High power, grating-free, cascaded Raman fiber lasers

There is significant interest in high power laser sources that can work outside the Yb emission band, for example, near 1.5um wavelength, for many applications like eye safe material processing, free space light transmission, pump sources for other lasers and LIDAR. Raman fiber lasers offer an excellent method to achieve high powers in a variety of wavelength regions [1]. In its conventional implementation, a high power Yb laser source is wavelength converted to the required output wavelength through a series of Raman Stokes shifts. In current Raman lasers, the choice of input and output wavelengths are fixed because of the fiber gratings they use for forming the cavity [2]. It is desirable to have a passive wavelength conversion module for wavelength conversion to a different band, which is color blind to the input wavelength. Recently, an interesting mechanism to avoid gratings in Raman lasers was proposed using distributed Raman feedback [3, 4]. However, prior work has been constrained in power scaling and efficiency, as there is no mechanism to terminate the cascade. In addition, as the wavelength conversion moves to longer wavelengths,(beyond 1.3micron), anomalous dispersion broadening of the stokes lines occurs. Owing to these reasons, the highest power achieved using such schemes is under lOW.