Study On The Characteristics Of Special-fiber-based Random Lasers Using Raman Amplification

Mirrorless random lasers formed by multiple scattering in a disordered gain medium have attracted significant interest among researchers over the past decades. In 1990 s, random lasers formed by disordered semiconductor or organic materials were reported experimentally. In 2010, a concept of a new type of random laser based on extremely weak random Rayleigh scattering(RS) in a standard Single-mode fiber(SMF) has been demonstrated by Turitsyn et al. The work has triggered many experimental and theoretical studies on Random distributed feedback fiber laser(RDF-FL). Besides, lots of attentions have been paid to their potential applications in fiber-optic communication and sensing.Furthermore, multiple scattering can be provided by other disordered gain medium, Dispersion compensated fiber(DCF), in which Random lasing(RL) is enhanced efficiently due to relatively large Raman gain and strong RS. Recently, studies have been focused on the issues of threshold reduction and high order emission of RDF-FLs.In this thesis, DCF is used to reduce threshold of RDF-FL; two different 2nd-order RDF-FLs are achieved; and RL by different combinations of SMF and DCF of varying length are investigate detailedly. In addition, a novel tunable multi-wavelength RDF-FL is realized.Firstly, random lasing characteristics through different combinations of SMF and DCF with a Raman pump(i.e., the DCF is mounted at the front or at the end of the SMF, the length of the SMF and DCF is also changeable) are studied in this paper. The results indicate that the position and length ratio of DCF and SMF influence the output of RDF-FL greatly(i.e., lasing threshold, spectrum stability and shape etc.). When DCF is located at a position where higher laser power is distributed, RL can be enhanced remarkably. These results are useful to reveal the role of DCF in RL and light amplification, and also provide theoretical support for flexible design and optimization of RDF-FL.Secondly, taking advantage of relatively strong Rayleigh scattering and Raman gain of DCF, three configurations to form efficient RDF-FL are proposed. Compared with the reported RDF-FL formed by SMF solely, lasing threshold and length of the proposed RDF-FL are effectively reduced through combination of DCF and SMF. In addition, FBGs with central wavelengths at the 1st and 2nd-order Raman Stokes wavelengths are also added to the hybrid SMF/DCF cavity to further reduce the lasing threshold, leading to realization of a new kind of 2nd-order RDF-FL. Besides, 2nd-order RDF-FL is realized in a SMF/DCF mixed cavity pumped bi-directionally from the fiber center, indicating that pump method is also an important factor to influence the performance of RDF-FL.Besides the above studies, it is suspected that RDF-FLs taking advantage of DCF will show emission characteristics different from RDF-FLs based on SMF, especially for relatively long DCF, while this hasn’t been addressed before. RL characteristics of 2nd-order random fiber laser formed solo by DCF are studied. It is found that the threshold of 1st-order RL is only 0.45 W. In addition, a special route to stable 2nd-order lasing is revealed, i.e., a special arc-shape output spectrum of the 2nd-order RL and three chaotic regimes during evolution from 1st to 2nd-order RL with increased pump power.Finally, multi-wavelength emission or wavelength tunable function based on RDF-FL has been realized. In this thesis, a novel all-fiber tunable multi-wavelength Raman fiber laser is achieved in a 50 km single-mode fiber connected to a Multi-channel wavelength tunable component(MCWTC), i.e., a F-P cavity combined with a long-period fiber gratings based Mach-Zehnder interferometer, that is obtained by micro-processing. Raman amplified Rayleigh scattering feedback provides the mechanism for random lasing which is split to equidistance wavelengths and tuned continuously from 1553.9 to 1565.4 nm by the MCWTC.The idea of using DCF provides a noval method to realize high order random fiber lasers. The results are useful to reveal the role of DCF in RL, and also provide theoretical support for ?exible design of RDF-FLs.