Transcript Using an Atomic Non-Linear Generated Laser Locking Signal to

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)
Objective
Introduction
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 nonlinear 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.
Conclusion
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.
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).
References
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.
Fabiano Queiroga, Wileton Soares Martins, Valdeci Mestre,
Itamar Vidal, 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.
Acknowledgements