Optimizing the optical imaging system by \emph{in-situ} imaging the plugged hole in the ultracold atoms

TL;DR

This paper presents a simple and efficient in-situ method to optimize optical imaging of ultracold atoms by imaging a plugged hole, reducing defocus artifacts and improving measurement accuracy.

Contribution

The authors introduce an in-situ imaging technique using a plugged hole to precisely optimize the optical system for ultracold atom experiments, enhancing image quality and measurement reliability.

Findings

Achieved imaging system optimization with 0.1 mm accuracy.

Demonstrated necessity for probing rubidium BEC with 5 ms TOF.

Validated the method's effectiveness over traditional focusing techniques.

Abstract

Optical absorption imaging has become a common technique for detecting the density distribution of ultracold atoms. The defocus effect generally produces artificial spatial structures in the obtained images, which confuses our understanding of the quantum systems. Here we experimentally demonstrate one method to optimize the optical imaging system by \emph{in-situ} imaging the plugged hole in the cold atoms. The atoms confined in a magnetic trap are cooled to tens of or several microkelvin by the radio-frequency evaporation cooling, and then are plugged using a blue-detuned laser beam, forming a hole in the center of the atomic cloud. We image the hole with a charge-coupled device (CCD) and quantitatively analyze the artificial spatial structure due to the defocus effect. Through minimizing the artificial structures by precisely adjusting the CCD position, we can optimize the imaging system with an accuracy of 0.1 mm. We also demonstrate the necessity of this method in probing rubidium BEC with a time of flight (TOF) of 5 ms. Compared to other methods in focusing the imaging system, the proposal demonstrated in this paper is simple and efficient, particularly for experimentally extracting large-scale parameters like atomic density, atomic number and the size of the atomic cloud.