2 May 2014 Superfocusing of high-M2 semiconductor laser beams: experimental demonstration
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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.
© (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
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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