Brian Neyenhuis and Dr. Dallin Durfee, Physics and Astronomy
Over the course of this grant we have completed the preliminary research for the first stabilization stage of our 657 nm diode laser. We have finished research on the theory behind our new extended cavity diode laser grating stabilization scheme, and were able to lock the laser to a high finesse cavity to determine the parameters necessary to build a next generation high speed lock circuit. We have also demonstrated the use of this laser in spectroscopic measurements of the Calcium clock transition.
A new stabilization scheme for an extended cavity diode laser was designed in our lab just prior to my joining the group [1]. I spent a large amount of my time exploring the parameter space of this laser to find the optimal configuration for mode-hop-free scanning. This was hindered by the presence of a reflective coating on the front facet of the diode, which caused an unwanted boundary condition. I was able to ignore the presence of this reflective coating by using current feed-forward to effectively change the position of the front facet of the diode at the same rate I scanned the frequency of laser. To determine this proper scan rate the laser is scanned with an arbitrary current feed-forward. If the laser mode hops by the free spectral range of the diode (in this case 45 GHz) forward, the current feed-forward rate is too fast, it if hopped backwards it is too slow. Then the current feed-forward rate is adjusted in the appropriate direction and the process is repeated until the optimal scan range is found. With this method I was able to match the current feed-forward to the other parameters that control scanning and was able to scan the laser up to 45 GHz mode-hop-free (the full free spectral range of the laser).
I was also in charge of developing and proving a simple geometric model to explain the laser’s operation. Despite its excellent scanning performance, we found no simple geometric model that adequately explains this scanning range and the optimal parameters I discovered. We believe that the lasers operation is a combination of several models proposed but is too complicated to make any simple predictions, so the parameters for optimum scanning must be determined experimentally. Although the theory behind the laser is not completely understood, reasonable optimization and operation are not difficult.
In addition to being able to scan mode-hop-free another important feature for a laser to have is stability. My passive stabilization of the laser made great improvements to its long-term stability. The laser now rests on two tiers of heavy aluminum plates each isolated by four squares of Sorebethane, a special type of rubber designed to damp out vibration. With this double isolation our laser drifts are reduced to several kHz an hour. The laser’s exceptional stability made it a good choice to measure the narrow transmission lines of an ultra high finesse cavity relative to the 1S0 – 3P1 Calcium clock transition. The results of this experiment have been submitted for publication in a peer reviewed journal [2].
In order to further stabilize the laser, active stabilization must be used. We are using a standard Pound-Drever-Hall lock [3] to generate an electronic error signal. In this setup light from the laser is sent into a high finesse cavity. By detecting at the light reflected by the high finesse cavity and using the proper electronic tricks we get a phase sensitive measurement of the frequency of the light vs. the transmission line of the cavity. This error signal has a steep slope that crosses zero when the laser is right on resonance with the cavity. This signal allows us to see which way the laser has drifted from the cavity resonance and correct the drift elcetronicly. I experimented with conventional lock circuits and made modifications to allow for better long term stability. To increase the servo bandwidth we designed a second generation lock circuit using new, faster, surface mount electronics. Working with this lock circuit has prepared us to make a final version and lock to an ultra high finesse cavity to attempt Hz level stability.
Research done under this grant has been presented at two national meeting [4,5] and has been published in a peer reviewed journal [2].
References
- R. Merrill, R. Olson, S. Bergeson, and D.S. Durfee, “Increasing the Output of a Littman-Type Laser by Use of an Intracavity Faraday Rotator,” Appl. Opt., 43, 3910-3914 (July 2004).
- C. J. Erickson, B. Neyenhuis, and D. S. Durfee, “A high temperature calcium vapor cell for spectroscopy on the 4s2 1S0 to 4s4p 3P1 intercombination line,” submitted to Rev. Sci. Instrum. (2005).
- R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M. Ford, A. J. Munley, and H. Ward, “Laser phase and frequency stabilization using an optical resonator,” Appl. Phys. B 31, 97-105 (1983).
- C. Erickson, B. Neyenhuis, J. Paul, G. Doermann, S. Bergeson, and D. Durfee, “Design and construction of a Ca/Sr interferometer.” DAMOP 2005
- B. Neyenhuis, R. Tang, and D. S. Durfee, “Progress Towards a Hz Stable 657 nm . Diode Laser.” OSA 2005.