Semiconductor saturable absorber mirrors (SESAMs) have become a key element of many ultrafast laser sources, enabling passively modelocked lasers with >100 GHz repetition rate or with >10 microJ pulse energy. Precise knowledge of the nonlinear optical reflectivity is required to optimize the SESAMs for self-starting passive modelocking at record high repetition rates or pulse energies. In this article, we discuss a new method for wide dynamic range nonlinear reflectivity measurements. We achieve a higher accuracy (<0.05%) with a simpler and more cost-efficient measurement scheme compared with previous measurement systems. The method can easily be implemented for arbitrary wavelength regions and fluence ranges.
In imaging and focusing applications, spherical aberration induces axial broadening of the point spread function (PSF). A transparent medium between lens and object of interest induces spherical aberration. We propose a method that first obtains both the physical thickness and the refractive index of the aberration inducing medium in situ by measuring the induced focal shifts for paraxial and large angle rays. Then, the fourth order angle dependence of the optical path difference inside the medium is used to correct the spherical aberration using a phase-only spatial light modulator. The obtained measurement accuracy of 3% is sufficient for a complete compensation as demonstrated in a model microscope with NA 0.3 with glass plate induced axial broadening of the PSF by a factor of 5.
A focused ion beam (FIB) was used to fabricate Fresnel phase lenses on optical fibre tips. The influence of dwell time and the scanning strategy to produce parabolic structures in silicon was investigated, because these parameters have a strong influence on the shape of the fabricated structures. The lens shape was characterized by atomic force microscopy and it was shown that the FIB does not roughen the surfaces. The optical performance of the lenses was characterized by scanning near field optical microscopy (SNOM) and the results were compared to simulations taking into account fabrication imperfections of the Fresnel lenses. At a wavelength of 840 nm a spot size of 740 nm FWHM was achieved.
Semiconductor saturable absorber mirrors (SESAMs) have become a key element of many ultrafast laser sources, enabling passively modelocked lasers with >100 GHz repetition rate or with >10 µJ pulse energy. Precise knowledge of the nonlinear optical reflectivity is required to optimize the SESAMs for self-starting passive modelocking at record high repetition rates or pulse energies. In this article, we discuss a new method for wide dynamic range nonlinear reflectivity measurements. We achieve a higher accuracy (<0.05%) with a simpler and more cost-efficient measurement scheme compared with previous measurement systems. The method can easily be implemented for arbitrary wavelength regions and fluence ranges.
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