Over the course of this grant, we have made considerable progress on our laser. We tested an anti-reflection coated diode in our setup, and although the anti-reflective coatings appear to be good, we did not see considerable improvement in scan range. However by experimenting with the geometry of the laser and implementing current feed-forward we were able to improve the scan range from about 9 GHz to greater than 50 GHz, more than sufficient for use in our atomic clock. We found a regular mode hop when attempting to scan over 50 GHz. We considered various etalons inside the cavity which could have caused this phenomenon (collimating lens, quarter wave plate, and other optics) but through extensive testing have found that the 50 GHz mode hop is due to the free spectral range of the laser diode. We could not find a way to eliminate this mode-hop, even with the use of an expensive anti-reflection coated laser diode. We spoke to the head of a company specializing in diode laser systems and found that this is a common problem even with the best anti-reflective coatings.
By using current feed-forward we were able to change the optical path length of the laser diode cavity allowing us to scan up to the free spectral range of the diode. With this extended scan range we tried various experiments to test out theoretical model of this laser and its scanning abilities. It was originally thought that the light coupling into the laser diode returned to the exact center of the diode at a slight angle, however the pivot point yielding maximum scan range predicted by this model could not be found. It appears that the light is coupling into the laser in a more complex manner than originally thought.
We have also done work to optimize the scan range of a simpler Littrow design diode laser, and are currently considering which design to use in our atomic clock. We currently have both types of lasers running in the lab and are using them to characterize a calcium vapor cell and develop a the lock circuit to be used in the atomic clock.
I have given the following presentations on this research:
B. Neyenhuis, D. S. Durfee, “Mentored Research in Laser Physics and Atom Interferometry” (Presentation to the BYU President’s Leadership Council), October 2004.
B. Neyenhuis, R. Merrill, S. Bergeson, D. S. Durfee, “Improving the Output Power of a Littman-Type Diode Laser,” OSA, October 2004.
C. Erickson, R. Olson, B. Neyenhuis, S. Bergeson, D. Durfee, “Progress towards a diode laser resonant with the 657 nm calcium intercombination line with Hertz-level stability,” DAMOP, May 2004.
D. Durfee, R. Merrill, R. Olson, S. Bergeson, “Improving the output of a Littman-Metcalf diode laser using an intra-cavity Faraday-effect isolator,” DAMOP, May 2004 (Talk presented by B. Neyenhuis, a student who started working on this project after the abstract had been submitted).
B. Neyenhuis, “Optimization of a New External-cavity Diode Laser Scheme,” BYU Spring Research Conference March 2004
I have also been accepted to give the following presentations:
C. Erickson, B. Neyenhuis, J. Paul, G. Doermann, S. Bergeson, D. Durfee, “Design and Construction of a Ca/Sr Atom Interferometer,” DAMOP, May 2005.
B. Neyenhuis, “A new diode laser stabilization scheme using an intra-cavity optical isolator,” BYU Spring Research Conference March 2005.