We report on the first demonstration of an all-fiber CW Raman laser based on a multimode graded-index fiber directly pumped by multimode fiber-coupled laser diodes. A joint action of Raman clean-up effect and mode-selection properties of special fiber Bragg gratings inscribed in the central part of the graded-index fiber core, results in high-efficiency conversion of a multimode (M~26) pump at 915 nm into a high-quality (M~2.6) output beam at 954 nm. About 50 W output power has been obtained with slope efficiency of 67%. The proposed development and integration of key multimode fiber technologies opens the door to new type of LD-pumped high-power high-beam-quality fiber lasers that may operate at almost any wavelength defined by available LDs.

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We demonstrate a high-power, high-efficiency Raman laser based on a 100 µm core graded-index (GRIN) fiber directly pumped by 915 nm multimode laser diode modules in all-fiber configuration. Optimization of GRIN fiber length and pumping scheme was performed. As a result, 62 W of CW power has been obtained at a wavelength of 954 nm with a slope efficiency of 85%. The joint action of Raman clean-up effect and mode-selection properties of special fiber Bragg gratings inscribed in the central part of the GRIN fiber core results in significant beam quality enhancement for the generated Stokes beam (  <  3) in comparison with that of the pump radiation (  >  30). In addition, the exact value of the operating wavelength near the Raman gain maximum was varied. To the best of our knowledge, this is the first study of the impact of a Raman gain spectral profile on the power, spectrum and beam quality parameter M2 of multimode Raman laser. It appears that they very much dependent on the spectral region of Raman gain at which we work, when the wavelength is selected by a fiber Bragg grating inscribed in a multimode GRIN fiber.

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Temperature calibrated piezoelectric resonances of internal acoustic vibration modes of a nonlinear-optical crystal during its heating by high-power laser radiation are used for noncontact measurements of both the non-uniform temperature distribution in the crystal volume and in the surrounding air. A novel notion of equivalent temperature of a crystal heated by laser radiation is introduced in laser physics. The true non-uniform crystal thermodynamic temperature at a given laser power is substituted by the measured equivalent crystal temperature, which is constant at that laser power. Using appropriate laser heating model the measured value of the equivalent crystal temperature allows one to calculate the unknown linear and nonlinear optical absorption coefficients as well as the heat transfer coefficient of the crystal with the surrounding air.