Second harmonics generation in fiber lasers
A review of the research results towards development of tunable fiber lasers operating in all visible range is provided. The main problems at frequency doubling of conventional fiber lasers are random polarization and large spectral width of their output radiation. For Yb-doped fiber lasers, an intra-cavity doubling in КТР with type II phase matching is applied providing conversion of both polarization components, second harmonics generation (SHG) in the range of 515-560 nm with smooth tuning of 20 nm has been obtained. For Raman fiber lasers (RFL), the SHG to 655 нм with tuning potential has been demonstrated. It has been shown that the broad RFL spectrum is converted efficiently due to the sum frequency mixing of different longitudinal modes having random phases.