In this work, we demonstrate an architecture to perform Raman-based power combining and simultaneous wavelength conversion of two independently controlled high-power Ytterbium doped fiber lasers operating at different wavelengths into a single laser line at the 1.5-micron band. Specifically, we have been able to achieve an in-band output power of ∼99W with a conversion of ∼64% of the quantum limited efficiency. This power combining is illustrated for different cases of the input wavelengths of the Ytterbium fiber laser. In each case, we have been able to demonstrate a power combining of >87 W in the final 1.5-micron band, with more than 85% of the fraction of the power residing in the final desired band.
Cascaded Raman lasers enable high powers at various wavelength bands inaccessible with conventional fiber lasers. However, the input and output wavelengths are fixed by the multitude of fiber gratings in the system providing feedback. In this work, we demonstrate a high power, tunable, grating-free cascaded Raman fiber laser with an output power of >30W and a continuous tuning range from 1440nm to 1520nm. This corresponds to the entire in-band pumping region of Erbium doped gain media. Our system is enabled by three novel aspects – A grating free feedback mechanism for Raman lasers, a filter fiber to terminate the Raman cascade at the required wavelength band and a tunable high-power Ytterbium doped fiber laser as input. In this work, the primary system is a novel, cascaded Raman conversion module which is completely color blind to the input pump source and does wavelength band conversion at high efficiency. In addition, the conversion module also provides high spectral purity of greater than 85% at the required wavelength by terminating the cascade using high distributed losses provided by specialty Raman filter fibers. Using a high-power Ytterbium doped fiber laser continuously tuned from 1060nm to 1100nm and Raman filter fiber with distributed loss beyond 1520nm, we achieve a continuously tunable 1440nm to 1520nm laser corresponding to 5th or 6th Raman Stokes shift of the input. To the best of our knowledge, the reported powers at these wavelengths have been the highest for tunable Raman fiber lasers and is currently only limited by the input power.
In this work, we report and analyse the surprising observation of a rainbow of visible colors, spanning 390nm to 620nm, in silica-based, Near Infrared, continuous-wave, cascaded Raman fiber lasers. The cascaded Raman laser is pumped at 1117nm at around 200W and at full power we obtain ∼100 W at 1480nm. With increasing pump power at 1117nm, the fiber constituting the Raman laser glows in various hues along its length. From spectroscopic analysis of the emitted visible light, it was identified to be harmonic and sum-frequency components of various locally propagating wavelength components. In addition to third harmonic components, surprisingly, even 2<sup>nd</sup> harmonic components were observed. Despite being a continuous-wave laser, we expect the phase-matching occurring between the core-propagating NIR light with the cladding-propagating visible wavelengths and the intensity fluctuations characteristic of Raman lasers to have played a major role in generation of visible light. In addition, this surprising generation of visible light provides us a powerful non-contact method to deduce the spectrum of light propagating in the fiber. Using static images of the fiber captured by a standard visible camera such as a DSLR, we demonstrate novel, image-processing based techniques to deduce the wavelength component propagating in the fiber at any given spatial location. This provides a powerful diagnostic tool for both length and power resolved spectral analysis in Raman fiber lasers. This helps accurate prediction of the optimal length of fiber required for complete and efficient conversion to a given Stokes wavelength.