Towards an optical double well trap and absorption imaging for an ultracold Caesium atom setup

Abstract

In this thesis, the absorption imaging setup of an ultracold Caesium atomsetup was improved by in corporating a ZL41 Wave 5.5 sCMOS camera and replacing the previously used lens system, resulting in an overall magnification of 1.25. Modulation transfer function measurements showed that the system is not diffraction limited, yielding a spatial frequency of 59±3 lp/mm at an MTF of 0.5 and a modulation of 0.23 ± 0.02 at the Nyquist frequency.The main part of this thesis focused on the realization of an optical setup that provides a controllable tool creating a continuous transition between a single and double mode beam inside an optical fiber. A P3-1550PM-FC-1 fiber was operated below its cut-off wavelength to excite the higher-order LP1,1 mode by varying the incident angle of the incoming laser beam. The output beam was focused by a microscope objective to create an optical trap of a few micrometers in size. Knife edgemeasurements yielded a value of 6.5±1.0 μm for the distance between mode maxima. Measurements revealed a transversal a symmetry between mode maxima near the beam focus despite symmetric far-field intensity profiles. These effects were attributed to the increased Gouy phase shift of higher-order mode contributions in the double mode beam during propagation through the focus. Adjusting the relative phase between mode contributions accordingly restored transversal symmetry but introduced a longitudinal asymmetry of mode maxima. Continuous transitions between single and double mode beams were demonstrated by simultaneously ramping two fiber-coupled acousto-optic modulators, which are used for amplitude modulation. Even splitting between modes as well as constant potential minima were observed. Finally, possible improvements and future implementation steps for integrating the optical double well setup into the main Caesium setup were discussed.