Teachspin - Optical Pumping: The Instrument

Optical Pumping

The basic features of the experimental set up of TeachSpin's Optical Pumping apparatus are shown in Figure 1. Rubidium resonance light from a heated rf discharge lamp is collimated by a plano-convex lens and passes approximately parallel through an interference filter, so that only the 795 mm line is transmitted. The light then passes through a linear polarizer and a quarter wave plate to produce a circularly polarized beam of light. This monochromatic, circularly polarized, light passes into the oven and through the rubidium vapor absorption cell. The diverging light is focused by a second plano-convex lens onto the photodiode detector.

The oven, with its absorption cell, resides inside two pairs of Helmholtz coils. The student must align the instrument so that the absorption cell’s axis (the light path) lies along the direction of the local Earth’s magnetic field. One Helmholtz pair is used to cancel the vertical component of the Earth’s magnetic field. The second pair is used to create a uniform horizontal magnetic field in opposition to the horizontal component of the Earth’s field. Transitions are induced among the atomic energy levels by the radio frequency magnetic field which is applied transverse to the optic axis. These transitions are observed as changes in the light intensity as measured by the photodiode optical detector.

The open construction of the Optical Pumping system allows students to manipulate the location of all the optical components. The can even insert the quarter wave plate and the linear polarizer in the wrong order which becomes an important learning experience.