Saturated Absorption Spectroscopy of Rubidium

Anne Marie March
SUNY Stony Brook
Optics Rotation Project 1, Fall 2000

Advisors: Prof. Hal Metcalf and Dr. John Noe



Project Goal

The goal of this project was to observe the Doppler-free D2 spectral lines of Rubidium using an extended-cavity diode laser. Rb has one electron outside of closed shells. The D2 spectral lines correspond to the transition of this electron from the 52S1/2 ground state to the 52P3/2 excited state. In conventional spectroscopy, a laser beam is sent through a sample of Rb gas, and its wavelength is scanned over the transition wavelength. The absorption spectrum observed has Doppler-broadened peaks due to the Maxwellian distribution of the Rb atoms' velocity. In order to resolve the hyperfine structure of the Rb atoms the technique of saturated absorption spectroscopy can be used. The diagram below illustrates the experimental set-up for this technique.


The Diode Laser

The diode laser is tuned to a particular wavelength by adjusting several parameters: the current to the laser, the temperature of the laser, and the angle of the diffraction grating which provides the feedback for the laser. This laser was constructed this past summer by Petr Liska. More information about it can be found at his webpage: http://laser.physics.sunys b.edu/~petr


The diode laser used in this experiment



Transition Parameters for lambda = 780.0 nm

The wavelength corresponding to the D2 transition in Rb is 780.0 nm. Finding the correct combination of laser temperature, current, and diffraction angle was a tedious process. A wavemeter was first used to measure the wavelength of the laser beam while the parameters were changed. Once close to the desired range, a CCD camera was used to look for the flouresence from the Rb cell while the parameters were systematically varied over small values. (The flourescence is in the infrared and is therefore not visible to the eye). Fortunately, I was able to tune this laser to the desired wavelength. Some values for the parameters are:

Diode Current = 75 mA    Diode Temperature = 15 C (approximate)

The pictures below show the Rb cell and the flourescence as seen by the CCD camera.




Diode Temperature Control -- Theory

In order to maintain a stable laser wavelength, it is crucial that the temperature of the laser be stable. The desired laser temperature is achieved and maintained by using a feedback circuit. In this experiment, the circuit cools the laser to the necessary temperature by means of a Peltier thermoelectric cooler, and then maintains that temperature by cooling the laser slightly more when it starts to heat up, and vice versa.

Diagram of a Thermoelectric Cooler.

Schematic Diagram of Feedback for Temperature Control.




interactive simulator

Complete Laser Setup with Temperature Control

The following picture shows the laser fully wired to the temperature controller and the current controller. The laser is sitting on an aluminum block insulated with foam.


Diode Temperature Control -- Experience with SDL Controller

The first temperature controller I used was an SDL-800. This box is actually designed to control the current and temperature for a specific diode laser set-up. However, I was able to use just the temperature controlling portion for my laser set-up.


SDL-800 Diode Laser Control Box

Unfortunately, the temperature stability using this controller was not good. After observing fluctuations in the temperature reading on the box that seemed to correspond with drifting on and off of the fluorescence, I investigated the temperature behavior more closely. This behavior is apparent in the following graph.

The data was taken using an AD590 chip to monitor the temperature of the laser and a Radio Shack digital multimeter with computer interface capabilities.


Temperature Oscillations with SDL Controller

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The oscillations are probably due to the difference between the thermal mass of the set-up being used and the thermal mass that the circuit was designed for. The time constants in the circuit are not appropriate for the set-up that I have.

The SDL controller has a option to by-pass the feedback elements in the circuit and to simply apply an external constant voltage to the thermoelectric cooler. I investigated this option. The temperature behavior is shown in the graph below.


Open Loop Behavior with SDL Controller

Although this method cooled the laser and maintained the temperature for about twenty minutes, the time required to wait for the laser to get to an equilibrium temperature was about half an hour. This would have made finding the voltage which would produce the correct temperature rather inconvenient.


Diode Temperature Control -- Experience with Bonn Controller

I obtained another temperature controller which was brought to Stony Brook by Robert Wynands from the University of Bonn, Germany. The time constants in this circuit can be altered if needed. Fortunately, I had to make only minor adjustments in the circuit to match the controller to my set-up.


Bonn Temperature Control Circuit

It appears that this controller is capable of achieving and stabalizing the temperature of the laser. This can be seen in the graph below.


Temperature behavior with Bonn Controller at a high set-point temperature


Unfortuantely, the attempt to cool the laser to the necessary temperature range was not sucessful. I believe this is due to an insufficient heat sink. Unfortunately, I did not have time to try to fix this.


Temperature behavior with Bonn Controller at a low set-point temperature




What I learned

Although I was not able to complete my original goal, I still feel that I learned a great deal.

  • I was able to show that this particular laser can be tuned to the Rb transition.
  • I learned the general idea behind saturated absorption spectroscopy.
  • I gained experience with temperature controlling circuits.
  • I gained a great deal of hands on experience in the optics lab, learning how to couple a laser to an optical fiber, learning about electronics, learning how to solder, learning how to align optics, etc.
  • And I learned how to make a web page!

Special Thanks

I would like to thank Dr. Metcalf and Dr. Noe as well as Christoph Affolderbach for his help with the Bonn temperature controller.