Analytic expressions are obtained for the spectrum of the light scattered when a collimated atomic beam is illuminated at right angles to its path by a CW monochromatic laser beam tuned to resonance with a two-level transition. The spectra vary depending on the portion of the interaction region from which the scattered light emanates. The spectrum of the light from a finite sub-region of the interaction volume is described as a function of the location and length of the sub-region, and of the intensity and frequency of the laser. Several interesting features related to the turn-on of the interaction as well as to the finite observation interval are found.
Highly collimated laser beam with excellent spatial quality is essential for quantum sensing experiments, where even small residual beam divergence can accumulate over distance and introduce significant systematic errors. In this article, we present the design and detailed characterization of a high precision laser beam collimator developed for a cold-atom Gravimeter system, capable of producing an expanded laser beam with a diameter of 16 mm while achieving microradian level collimation accuracy through a five-degree-of-freedom (5-DOF) adjustment mechanism. The beam quality is evaluated using an ISO11146 compliant beam propagation measurement combined with Gaussian beam analysis to extract key parameters, including the beam waist, divergence angle, Rayleigh length, and beam quality factor
M2. The measured divergence angles of 0.006° (105 micro-radian) along the
x axis and 0.007° (122 micro-radian) along the
y axis confirm stable and well controlled collimation over long propagation distances. The demonstrated collimation architecture and characterization methodology provide a robust and scalable solution for cold-atom Gravimetry and other precision optical applications that require stable, high quality laser beams maintained over extended distances.