Daniel Thrasher and Dr. Scott Bergeson, Physics Department
Laser frequency control is imperative for cold atom experimentation. In our lab we use finely tuned lasers to optically cool and trap calcium atoms. Our ability to successfully trap atoms is highly dependent on the stability of the lasers we use to cool them. After trapping the atoms we use more finely tuned lasers to reveal how cold the atoms are and how quickly they leave our trap. Previous to this work our lasers were limited to 20 MHz precision. By installing a femtosecond laser as a standard by which to stabilize the other lasers in our experiment we successfully demonstrated 500 kHz precision (a factor of 100 improvement). In the sections that follow I will discuss how we implemented this system and the new physics we demonstrated as a result of our increased laser precision.
Optical metrology has been an active research field since the invention of the laser. Recent advances have lead to the production of a compact system capable of stabilizing laser wavelengths from 500 to 1100 nm. This audacious feat is accomplished by stabilizing the comb modes of a femtosecond oscillator and then comparing those modes with as many continuous lasers as your power and/or bandwidth permit.
Femtosecond oscillators are lasers whose pulse duration are extremely short and (by Heisenberg’s uncertainty principle) therefore significantly broad in frequency bandwidth. In light of Fourier Analysis this implies that the frequency components of such a laser are discrete like the teeth of a comb. The spacing of this comb is determined by the repetition rate of the laser. The repetition rate of the laser is controlled by making fine adjustments to the length of the laser cavity. By stabilizing the repetition rate using a negative feedback amplifier we can produce from one laser what would require 18,000 individual continuous lasers! We stabilize the repetition rate of the laser by locking it to a very stable clock. In our case we lock it to a Global Positioning Satellite (GPS) disciplined Rubidium oscillator, which is merely a clock that receives updates every second from local orbiting satellites thus ensuring its high precision.
In addition to stabilizing the repetition rate or comb spacing, one must also stabilize the phase or overall translation of all the comb modes as a whole. As the laser operates in time, this phase is inconsistent from pulse to pulse due to power fluctuations. We stabilize this phase by implementing another negative feedback loop to control the power of the laser.
Once our frequency comb is stabilized we can then overlap the various continuous wave lasers we wish to lock with the femtosecond laser and produce a beat note. A beat note is a difference frequency. These difference frequencies can be locked as well, meaning that our lasers are now not changing relative to the frequency comb. All the stability of our Rubidium oscillator is transferred to optical frequencies.
We used these highly stabilized optical frequencies to demonstrate for the first time laser cooling of calcium ions in a neutral ultra cold plasma. Although laser cooling of ions is a well developed technique, which garnered the 2012 Nobel Prize in physics for David Wineland, laser cooling of ions in a neutral plasma had yet to be successfully demonstrated. When atoms are Doppler cooled they slow down due to absorption of momentum from light propagating opposite to the atom’s direction of motion. The laser light absorbed by an atom changes with its velocity. We used our new precise laser system to show that we could significantly cool the calcium ions in a neutral plasma by Doppler cooling techniques. Although other research groups have attempted to demonstrate this before, our superior laser precision enabled us to successfully demonstrate the technique. A paper describing these data will be prepared for publication within the next several months.
The purpose of our lab’s research is to study how plasma systems behave when the coulomb interaction of the nearest neighbor ions dominate over the thermal energy of the ions. A plasma which meets this criterion is referred to as “strongly coupled”. Such information has direct application to increasing our understanding of laser driven nuclear fusion and many interstellar plasmas. Our new ability to cool the ions of our neutral plasma enables us to further control and test the parameters of our plasma. We plan to test how cooling the ions increases the strong coupling of ultra cold plasmas.
We are also interested in better understanding why our laser precision is limited to 500 kHz. We determine the precision of the laser system by doing spectroscopy on ultra cold calcium atoms and comparing our resonance frequency with those published in the literature. To date, experimentation suggests that our precision measurement is limited by stray magnetic fields within our vacuum chamber which limits the validity of our spectroscopic standard. It is possible then that our system is more precise then we currently have the means to measure.
In addition, many Calcium absorption resonances have been measured with a precision less than our currently demonstrated limit. We plan to repeat these measurements and update the literature with more precise values.
We have successfully demonstrated a stabilized optical frequency comb capable of stabilizing two independent lasers to a precision of 500 kHz. We utilized this technique to demonstrate Doppler cooling of calcium ions in a neutral plasma. This new technique enables us to perform many new experiments which will have an important impact on the atomic physics community