2 May 2014 Superfocusing of high-M2 semiconductor laser beams: experimental demonstration
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Abstract
The focusing of multimode laser diode beams is probably the most significant problem that hinders the expansion of the high-power semiconductor lasers in many spatially-demanding applications. Generally, the ‘quality’ of laser beams is characterized by so-called ‘beam propagation parameter’ M2, which is defined as the ratio of the divergence of the laser beam to that of a diffraction-limited counterpart. Therefore, M2 determines the ratio of the beam focal-spot size to that of the ‘ideal’ Gaussian beam focused by the same optical system. Typically, M2 takes the value of 20-50 for high-power broad-stripe laser diodes thus making the focal-spot 1-2 orders of magnitude larger than the diffraction limit. The idea of ‘superfocusing’ for high-M2 beams relies on a technique developed for the generation of Bessel beams from laser diodes using a cone-shaped lens (axicon). With traditional focusing of multimode radiation, different curvatures of the wavefronts of the various constituent modes lead to a shift of their focal points along the optical axis that in turn implies larger focal-spot sizes with correspondingly increased values of M2. In contrast, the generation of a Bessel-type beam with an axicon relies on ‘self-interference’ of each mode thus eliminating the underlying reason for an increase in the focal-spot size. For an experimental demonstration of the proposed technique, we used a fiber-coupled laser diode with M2 below 20 and an emission wavelength in ~1μm range. Utilization of the axicons with apex angle of 140deg, made by direct laser writing on a fiber tip, enabled the demonstration of an order of magnitude decrease of the focal-spot size compared to that achievable using an ‘ideal’ lens of unity numerical aperture.
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G. S. Sokolovskii, G. S. Sokolovskii, V. Melissinaki, V. Melissinaki, V. V. Dudelev, V. V. Dudelev, S. N. Losev, S. N. Losev, K. K. Soboleva, K. K. Soboleva, E. D. Kolykhalova, E. D. Kolykhalova, A. G. Deryagin, A. G. Deryagin, V. I. Kuchinskii, V. I. Kuchinskii, Evgeny A. Viktorov, Evgeny A. Viktorov, M. Farsari, M. Farsari, W. Sibbett, W. Sibbett, E. U. Rafailov, E. U. Rafailov, } "Superfocusing of high-M2 semiconductor laser beams: experimental demonstration", Proc. SPIE 9134, Semiconductor Lasers and Laser Dynamics VI, 91341N (2 May 2014); doi: 10.1117/12.2052483; https://doi.org/10.1117/12.2052483
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