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

agreementkittensSemiconductor

Nov 2, 2013 (3 years and 7 months ago)

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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)


Conclusion

References

Acknowledgements

Introduction

Theory and Methods

Objective

Results

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.

Figure 1. This diagram depicts the setup used to obtain
frequency
dependent signals,
a dispersive
ANGeLLS and a saturated
absorption signal.

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 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).

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.

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.

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.

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
.

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).