Saturated Absorption Spectroscopy

The What.

A large part of part of scientific research involves the principles of optical and atomic physics through laser-driven saturated absorption spectroscopy. In this experiment a single laser is split to provide both a pump and probe beam to examine the absorption spectra of a vapor cell containing primarily rubidium85 and rubidium87 as shown:

The How.

Initially, one must check for fluorescence within the vapor cell to ensure that the laser intensity is tuned properly. Very precise control of the laser is provided by frequency and temperature (wavelength) locking circuits. Once fluorescence is achieved adjustments are made to the frequency of the laser (adjusting the voltage to the laser’s PZT) to “scan” over the spectra lines of the ground and excited states of rubidium. These lines are observed on a digital oscilloscope. One small change is to be noted: the photodiode in this experiment had two active regions and a subtraction circuit attached to it. This allows for the account of Doppler shifts within the vapor cell while scanning to provide clearer spectra lines.

Once several lines were observed, the best observable spectra lines are recorded. See below (Standard Transition Level Schema) for expected states to be visible in the experiment:

Next, the vapor cell is switched out with another wrapped in coils of wire attached to a power supply in order to observe the effects of a magnetic field generated by the Zeeman shifter on the vapor cell. Once the Zeeman shifter is powered, another absorption line is recorded and saved once again for comparison to the preliminary lines where a splitting of the lines should be observed. See below for a visual representation (Zeeman Level Schema) of the splitting expected.

The Numbers.

Rb85 5P_3/2 Spectrum Lines:

Table 1: No Effect, Rb85 5P_3/2

Zeeman Effect Spacing

The Outcome.

The exercise of this experiment is particularly useful in determining the proper operating frequencies of a laser allowing an individual to create a magneto optical trap or MOT. Principally, one should be able to find the frequencies at which it is possible to slow rubidium atoms both when under the influence of the Zeeman effect and otherwise. Although after considerable trial and error in the short lab window given, we were able to properly align the optical setup with the diode to achieve both a pump and probe beam providing fluorescence, it was difficult initially to locate the spectra lines of the rubidium in the vapor cell. After finally getting clean lines without the Zeeman effect, an average frequency spread was calculated for the Rb85 5P_3/2 transition lines to be approximately 11 GHz with an average spreading of 5.6 GHz/Volt. These findings seem to be valid. However, once the effect was introduced severe noise was clearly visible in the signal on the oscilloscope most likely from building vibration and potentially interference picked up through the effect’s wires. In order to combat this issue, the digital oscilloscope’s snapshot feature was used to prevent movement of the lines while recording data. This may have negatively impacted the Zeeman effect data even though multiple snapshots were taken, as the spectrum was constantly moving on the oscilloscope.

Thus, the first half of the experiment was useful in observing spectra lines. Noise and incorrect data in the second part of the experiment involving the effect resulting in 40% error is unsatisfactory to make any conclusions about the effects of the Zeeman effect on splitting spectra lines. However, the splitting is indeed visible. If the experiment were to be repeated, greater care eliminating noise and alignment should be taken, as well as increased resolution on the oscilloscope in order to achieve a better comprehension of the Zeeman effect. It should be noted that this research was conducted as an undergraduate experiment and the results are not to be considered valid as the experiment was repeated later with greater precision and verification. If there is any incorrect information presented please feel free to send me an e-mail, I’d appreciate it!