Using an Atomic Non-Linear Generated Laser Locking Signal to Stabalize Laser Frequency

1. Using an Atomic Non-Linear Generated Laser Locking Signal to Stabalize Laser Frequency Gabriel Basso (UFPB), Marcos Oria (UFPB), Martine Chevrollier (UFPB),Thierry Passerat de Silans (UFPB), Kartik Pilar (SUNY) Introduction Objective In order to use lasers to create a super-cold cloud of atoms, through processes such as Doppler cooling, a laser must be tuned to very specific wavelengths. To create a laser system with constant wavelength, a feedback system must be used. For our system, an atomic, non-linear generated laser locking signal (ANGeLLS) is used to modulate the current through the laser and correct fluctuations in the emitted wavelength of the laser. We are using the non-linear medium properties of rubidium to monitor the frequency of a laser, and lock the wavelength of the laser to lengths that correspond to transition energies of the excitation states of rubidium. Theory and Methods Rubidium vapor is known to be a non-linear media in terms of its index of refraction with respect to light intensity. With this knowledge, we divert part of the power of a semiconductor diode laser, which is modulated by a function generator, through a lens and then a heated rubidium cell. The lens focalizes the light on the cell increasing the non-linear effects. The Gaussian intensity profile of the beam induces a refractive index gradient that acts as a lens whose focal length depends on the laser frequency. This allows the non-linear properties of the rubidium cell to create a dispersive signal in a photo detector, which is placed after an aperture, due to changes in the frequency causing the cell to act as a lens with a changing focal point. Using the dispersive signal through the photo detector, an electronic feedback circuit stabilizes the frequency of the semiconductor laser by modulating current. When the feedback system is turned on, the function generator is turned off. Also, to maintain an understanding of the wavelength of the laser, and it’s fluctuations, a separate portion of the power of the beam is diverted towards an unheated rubidium cell. This time, no lens is used. Instead of a dispersive signal, an absorptive signal is received by the photo detector. However, by saturating the cell with a counter-propagating beam, hyperfine transitions can be seen, which is known as saturated absorption spectroscopy..These hyperfine transitions can be isolated by making an amplitude modulation in the pump beam and a homodyne detection using a lock-in amplifier, and then used as a reference to monitor the stability of the ANGeLLS system. Figure 1. This diagram depicts the setup used to obtain frequency dependent signals, a dispersive ANGeLLS and a saturated absorption signal. Figure 2. This graph shows, from bottom to top, the function modulating the current of the laser from the function generator, the saturated absorption signal with hyperfine transitions, and the dispersive signal from the ANGeLLS system. The transition used for stabilization is circled. Figure 3. This graph shows the saturated absorption signal after the lock-in amplifier is used (bottom), along with the hyperfine transitions that have been isolated by the lock-in amplifier(top). Results We observed that by using the ANGeLLS system, the frequency of the laser was stabilized to within 40 MHz, which corresponds to a change in wavelength on the order of 10-5 nm. Previously, the change in wavelength was much greater as shown in Figure 5. Without the feedback system, the signal seems to drift, even leaving the hyperfine transition. The changes in frequency cause a change in the index of refraction of the ANGeLLS cell, and therefore cause fluctuations in the voltage in the signal from the photo detector. Figure 4. This graph shows the signals from the ANGeLLS system (bottom), and the isolation signal from the lock-in amplifier of the hyperfine transition(top) when there is feedback. Figure 5. This graph shows the signal from the ANGeLLS system when the feedback system is off. Shown is the signal from ANGeLLS (top) and the hyperfine transition (bottom). Conclusion References Acknowledgements B. Farias, T. Passerat de Silans, M. Chevrollier, and M. Oria (2005). Frequency bistability of a semiconductor laser under a frequency-dependent feedback. Physical Review Letters, 94(17), 3902-3905. FabianoQueiroga, WiletonSoares Martins, ValdeciMestre, ItamarVidal, Thierry Passerat de Silans, Marcos Oria, and Martine Chevrollier. Laser stabilization to an atomic transition using an optically generated dispersive line shape. Submitted, awaiting publication. Although the laser appears to be frequency stable, there is still room for improvement. Most improvement can be made by obtaining a better ANGeLLS signal by reducing noise from table movement, sounds, and vibrations from the air conditioner. In the future, three of these laser systems will be used in an experiment to cool a cloud of rubidium atoms to temperatures of a few hundred millikelvin by a process of Doppler laser cooling.