Change of optimum conditions of second harmonic generation of spatial limited laser beams in nonlinear crystals U. K. SAPAEV*, I. A. KULAGIN AND T. USMANOV Laser-Matter Interaction Laboratory, NPO Akadempribor, Academy of Sciences of Uzbekistan, Tashkent 700125, Uzbekistan (*author for correspondence: E-mails: sapaev@mail.tps.uz, susman74@yahoo.com) Received 16 May 2003; accepted 28 August 2003 Abstract. Frequency doubling of spatially limited laser beams in nonlinear crystals is investigated theo- retically. It was shown that the major factor, which reduces the second harmonic generation (SHG) eﬃciency in this case, is a diaphragm aperture eﬀect, caused by inﬂuence of extraordinary polarized laser beams. In that case a spatial shift of mutually orthogonally polarized beams of the incident waves relative to each other in perpendicular direction of wave transmission at the entry to crystals lead to increase the eﬃciency of SHG of limited beams. Key words: KDP crystal, nonlinear crystals, second harmonic generation, spatial limited laser beams, walk-oﬀ angle The anisotropy of nonlinear crystal exerts an essential inﬂuence on eﬃciency of various frequency conversion processes such as, second harmonic gener- ation (SHG), cascade generation of harmonics, frequency mixing, sum-fre- quency, parametrical generation and ampliﬁcation of light etc., in spite of realization of phase-matching conditions of the interacting spatial limited laser beams. Since, real laser beams, limited in spatial sizes unlike plane light waves, may lead to diaphragm aperture eﬀect (DAE) (‘walk-oﬀ’ angle) (Dmitriev and Tarasov 1982, p. 99). It is well known this eﬀect is only characteristic of extraordinary polarized beams, because their direction of propagation of the wave phase generally does not coincide with the direction of wave energy. The inﬂuence of the DAE determines the SHG eﬃciency, if crystal length is greater than so-called aperture length (Dmitriev et al. 1999, p. 37). The inﬂuence of DAE on the eﬃciency of SHG is varied depending on a physical property of beams. For instance, if we use a limited laser beam with the plane phase front the eﬃciency of SHG is limited by inﬂuence of DAE, caused by ‘walk-oﬀ’ angle of extraordinary beams. In contrast to this case, if the SHG process is carried out in a ﬁled of the divergent limited laser beams, the eﬃciency of SHG is limited by inﬂuence of both DAE, and angular aperture eﬀect, resulting from a phase mismatch of squinted beams Optical and Quantum Electronics 35: 1311–1315, 2003. Ó 2003 Kluwer Academic Publishers. Printed in the Netherlands. 1311