Linewidth of high-power fiber lasers Marc-André Lapointe [a-b], Michel Piché [b] [a] CorActive High-Tech, 2700 Jean-Perrin, suite 121, Québec, Canada, G2C 1S9 [b] Centre d’optique, photonique et laser, Université Laval, Québec, Canada G1V 0R6 ABSTRACT In this work, we examine how the linewidth of high-power Yb-doped fiber lasers changes as a function of laser power. Four-wave mixing between the various longitudinal modes of the laser cavity tends to broaden the laser linewidth, while Bragg reflectors have a narrow bandwidth that limits the extent of this broadening. An analytical model taking into account these effects predicts that the laser linewidth scales as the square root of laser power, in agreement with numerical simulations [1]. This model has been previously validated with a low-power Er-doped fiber laser [1] and with Raman fiber lasers [2]. In this paper, we compare the measurements taken with Yb-doped fiber lasers at power levels ranging from a few watts to hundreds of watts with the model. The broadening of high-power fiber lasers deviate from the model. Experimental data show that the linewidth broadens as a power function (between 0.5 to 1) of the laser power. A simple modification of the model is proposed which fits all the experimental data. Keywords: laser linewidth, high-power CW fiber lasers, spectral broadening, Yb-doped fibers.
1. INTRODUCTION High-power fiber lasers (HPFL) have generated a lot of attention over the past few years. Multi-kilowatt implementation of multimode HPFLs and kilowatt implementation of singlemode (SM) HPFLs have been achieved and commercial products are already available on the market [3]. The state-of-the-art demonstration of single mode operation of a fiber laser is currently at 6 kW [4] and prediction of SM operation at power levels exceeding 30 kW has been made [5]. The capability of fiber lasers to maintain an excellent beam quality at high power is one of their main advantages. However, there is little concern of the laser linewidth of HPFLs as they are generally used for processing of materials that have broad absorption bands. The spectrum of fiber lasers is known to broaden with power and this phenomena is often incorrectly associated to the heating or the spectral deformation of the fiber Bragg gratings (FBG). The spectral broadening of fiber laser results from the four-wave mixing (FWM) between its 10 longitudinal modes. An electronic spectral analyzer will reveal the presence of those modes by showing the Fabry-Perot axial mode spacing of the laser. V. Roy et al. [1] have validated an analytical model of the laser spectrum at low power (4W) was 50 times higher than with the standard configuration where . Also, the linewidth did not coincide with the center of the HR bandwidth, causing a decrease of the effective HR width to approximately 0.5 . The cavity parameter and the linewidth broadening curve for this configuration are shown in Figure 10. There is is doubled. However, the power dependence is now 0.5. more broadening in this case because Δ f
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Wavelength (nm) 1070 -15
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Figure 9: Backward spectrum at the pump combiner as function of output power of a 6/125 laser in which a flat cleave is replacing the LR FBG.
A γ σ σ
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Figure 10: Output spectral broadening of a 6/125 laser with
0.54. A flat cleave is used to replace the LR FBG.
4. CONCLUSION Experimental results on the spectral broadening of high-power fiber laser (HPFLs) were compared with an existing theory of four-wave mixing (FWM). It was shown that the FWM is the dominant broadening mechanism in HPFLs. But experimental data deviate from the theoretical square-root law relationship that was previously validated for a low power erbium ring fiber laser [4] and long Raman fiber lasers [5]. FWM increases the linewidth of HPFLs as a function of power with an exponent between 0.5 and 1. The theoretical model was modified to include the fitted value of exponent . This modified model is in good agreement with all experimental data. A large spectral broadening affects slope efficiency and causes leakage of power through the high-reflectivity mirror.
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ACKNOWLEDGEMENTS This work has benefited from the financial support from CorActive High Tech and Laval University. Marc-André Lapointe was supported by an FQRNT-NSERC scholarship awarded through the BMP “Photonics” program managed in collaboration with the Canadian Institute for Photonic Innovations (CIPI).
REFERENCES [1] Roy, V., Piche M., Babin, F., Schinn, G., “Nonlinear wave mixing in a multilongitudinal mode erbium-doped fiber laser,” Optics Express, vol. 13, 6791-6797 (2005). [2] Babin, S., Churkin, D., Ismagulov,A., Kablukov, S., Podivilov, E., “Four-wave-mixing-induced turbulent spectral broadening in a long Raman fiber laser,” Journal of the Optical Society of America B (Optical Physics), vol. 24, 1729-38 (2007). [3] IPG Photonics, http://www.ipgphotonics.com. [4] Gapontsev, D., “6kW CW single mode ytterbium fiber laser in all-fiber format”, 21st Annual Solid State and Diode Laser Technology Review, 258 (2008). [5] Dawson, J.W. , Messerly, M.J. , Beach, R.J., Shverdin, M.Y. , Stappaerts, E.A. , Sridharan, A.K., Pax, P.H., Heebner, J.E. , Siders, C.W. , Barty, C.P.J. , “`Analysis of the scalability of diffraction-limited fiber lasers and amplifiers to high average power”, Optics Express, vol. 16 no. 17, 13240-13266 (2008). [6] Agrawal, G.P, [Nonlinear Fiber Optics], Academic press, San Diego, 447-451 (2001). [7] Garcia, H., Johnson, A.M., Oguama, F.A., Trivedi, S., "Pump-induced nonlinear refractive-index change in erbiumand ytterbium-doped fibers: theory and experiment," Optics Letters. vol. 30, 1261-1263 (2005)
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