Trapping ions presents a bit of a conundrum for frequency stabilizing lasers: How does one lock a laser to a line in some ion when you can’t access that line without the laser already being locked to let you trap the ion in the first place? Different groups solve the problem in different ways; one can access some transitions in ions using atomic discharge lamps, or find a suitable line in another system like tellurium or iodine. But, our group has always used a scanning Fabry-Perot cavity to transfer the stability of some reference laser (usually a commercial stabilized HeNe) on to our ECDLs.* So long as one does not need an exceptionally tight lock, this technique is extremely general, and a single setup can be used to lock many ECDLs to a single reference laser.
One key to getting a stable, drift-free lock is to enclose the FP `transfer cavity’ in a hermetically sealed box. In the absence of such an enclosure the dispersion of air leads to the frequency of a locked laser drifting relative to the HeNe reference even while its in-air wavelength is stabilized; for blue lasers locked to the 633 nm HeNe line this can lead to frequency drifts of a few hundred MHz as the atmospheric pressure and humidity varies from day to day. Sierra Jubin ’17 cataloged this issue in her thesis:
Here the red trace is the locked FP transmission fringes from the HeNe lasers, while the black trace is the locked fringes from the 423 nm ECDL we use for photoionizing neutral Ca. The dashed black lines show where we had to lock the laser to optimize photoionization as a function of the weather, with the numbers representing atmospheric pressure in inches of Hg.
Over the last year we implemented a new FP transfer cavity, building a fused silica/stainless steel assembly with lengths matched to the thermal expansion coefficients of the two materials to make the cavity athermal overall.** We also installed the cavity inside a CF tee to isolate it from atmospheric pressure/humidity variation, hoping to solve the problems Sierra observed with drifting ECDL lockpoints as the weather varied:
Even so, we tended to see substantial drifts of the cavity fringes from day to day. Turns out that our epoxy-sealed windows were leaking. After (finally) fixing the problem the cavity now performs as desired:
Here we’re watching (in blue) the drift of the frequency-stable HeNe laser relative to the cavity on a day when the laboratory temperature control was disabled and the atmospheric pressure varied by about 1% (not shown). The lab temperature drifted by about 5C; the cavity temperature (in red) is loosely stabilized such that it follows the lab temperature but with ~15x reduced variation. With the athermal cavity and the CF Tee properly sealed the voltage required to keep the HeNe fringe at the same place in the cavity barely moves over eight hours despite the large fluctuations in lab temperature and atmospheric pressure; the 0.02 V fluctuation corresponds to roughly 1/400 of a cavity free spectral range.
Needless to say, this improvement to the laser lock will improve the long-term stability of our locked lasers, vastly improving our ability to trap ions for extended periods without having to “chase” the Doppler cooling lines.
*This is a technique that Prof. Doret actually learned as an undergrad at Williams, working on Tl spectroscopy with Prof. Majumder.
**This design is based off of one from the DeMille group at Yale; see John Barry’s thesis for details.
