Fiber lasers have progressed quickly in recent years. Yb-doped fiber laser at 1 mm has reached 20 kW in the case of diffraction-limited beam quality, and 100 kW in the case of multimode output. However, the spectral coverage of rare earth doped fiber lasers are limited by the available transitions. Stimulated Raman scattering in optical fiber is an efficient way to extend the spectral coverage of fiber lasers.

Prof. Yan Feng and colleagues at Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences have been working on high power Raman fiber lasers and applications for years. Recently, they proposed an integrated ytterbium-Raman fiber amplifier architecture, which solves the main technical bottleneck for further power scaling of Raman fiber lasers. With the architecture, up to 580 W singlemode linearly-polarized and 1.3 kW near-singlemode Raman fiber laser are demonstrated.

In the proposed architecture, standard high power Yb-doped fiber amplifiers are seeded with multiple lasers, whose wavelength separations equals to the Raman shift. The laser in the middle of the Yb gain spectrum gets amplified first, and transfers power to the longer wavelength lasers in the following optical fiber by stimulated Raman scattering. In a proof of principle experiment, 300 W Raman fiber laser at 1120 nm is demonstrated. Shortly after that, 580 W singlemode linearly-polarized and 1.3 kW near-singlemode Raman fiber laser are achieved with gain fibers of larger cladding diameter (400 mm). These results are published on Optics Letters and Optics Express [Opt. Lett. 39, 1933-1936 (2014); Opt. Express 22, 18483 (2014)]. Since all current high power ytterbium fiber lasers have a master oscillator power amplifier structure, the proposed architecture can be applied and scales the Raman fiber laser output further. Primary numerical simulation shows that the method has potential in generating over kilowatt laser at any wavelength within 1~2 mm.

The works were supported by the National Natural Science Foundation of China (No. 61378026).