Optical EngineeringCharacterization and preparation of focused laser beams
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We discuss the possibilities of a far-field beam profiling technique for the determination of the near-focal profiles of laser beams focused beyond the paraxial approximation. The analysis is based on the rigorous vectorial Fourier synthesis of the fields and includes polarization effects. The wave-vector distribution can be calculated either a priori using the radius of the impinging laser beam and the data of the objective, or by obtaining its parameters from profiling at different z positions far behind the objective. For such measurements we designed a profiling system suitable for in situ application in microscopy. The device consists of a fiber optic taper bonded onto the chip of a CMOS camera, allowing taking pictures in the working medium in planes sufficiently close to the focus. The near-focal profiles obtained by these two approaches show good agreement, but only if the measured effective numerical aperture NAeff is used. We have further readdressed the effect of filling the objective pupil on the near-focal fields. The small improvement in focusing by the commonly practiced overfilling is hardly worth the significant loss of power. Moreover, overfilling above an optimal beam radius aopt results in an increase of aberrations when focusing through an optical interface.