Transcript poster_landscape_eng

DSP Based Controller for
Very Accurate LASER
Moskovich Moran
[email protected]
Introduction
Method
Physics of quantum systems and the development of
matter-wave quantum technology is at the peak of
modern research.
To achieve the high accuracy needed, a commercial
LASER diode is used together with an external cavity.
This external cavity is made using a diffraction grating
and a piezoelectric actuator. The grating is a mirror that
returns part of the beam back to the commercial laser’s
internal cavity from a few centimetres
distance,
resulting in a very high accuracy laser beam.
In the Bose-Einstein Condensate (BEC) lab at the
Weizmann Institute of Science, rubidium atoms are
brought to the
Bose-Einstein condensate (BEC) state.
This state predicted by Satyendra N. Bose and Albert
Einstein in the 1920’s enables the study of phenomena
that occur at a very small scale.
To reach this new state, the atoms are brought to a very
cold temperature of 170 nano Kelvin.
Cooling the atoms is achieved by pointing a very accurate
LASER from beam from all six directions to the center.
&
Fishelzon David
[email protected]
implementation
The problem is that this scheme is sensitive to acoustic
noise. Surrounding noise vibrates the mechanical
structure holding the cavity components, thus causing
the length of the cavity to change, generating changes
in the output laser beam frequency.
FxLMS – Filtered X LMS
Block diagram of the
DSP based controller.
The microphone that
is located near the
cavity, is receiving the
acoustic noise, and
the
DSP
is
compensating for this
noise, in real time.
Noise1
DF FIR
u
x
DF FIR
DF FIR
y
u
SYSTEM1
e
d
e
NOISE
LMS
y
SYSTEM
W
Piezo Out
Lock Signal
BLACK fin
Simulated
System
3 1
4 2
OUT (J4)
NOISE
3
RIGHT (Red)
4
IN (J5)
Simulation Output
Controller
This project is a feasibility proof of the implementation of
the FxLMS algorithm as a noise reduction controller for
an accurate laser output frequency.
Future work could implement this DSP algorithm in a real
world system but, in our opinion, a faster DSP is needed.
Also faster analog to digital and digital to analog
converters than the ones we used on the EZ-KIT
demonstration board are required.
A faster DSP could also enable the use of sophisticated
pattern recognition algorithms to enable automatic finding
and locking on the desired absorption line of the laser’s
output frequency detector.
LEFT (White)
1
2
Noise In4
System In3
External Cavity Beam Splitter
LASER
Future work
Piezo
Out
- Xn
Input
Output - Yn
3 1
4 2
1
e2
n
LEFT (White)
3
RIGHT (Red)
4
error in
Probe
Pump
Rubidium Cell
Detector
Grating
LASER
Piezo
Results
Conclusion
Measurements of the transfer function of the system and
the transfer function of the noise reviled a very close to
the theory picture :
The FxLMS algorithm for noise cancellation is suitable for
reducing the laser frequency output noise thus enhancing
its stability and quality.
For more information on:
FxLMS Noise cancelling algorithm, Blackfin DSP
implementation and more ideas for future projects.
Contact:
Algorithm
Hot atoms are moving, thus having their absorption line
frequency shifted by the Doppler effect. This shift makes
the hot atoms move in the opposite direction –resulting in
them stopping – hence cooling.
Aim
The aim of the project is a feasibility test for using the
FxLMS algorithm as a control algorithm of the LASER
output frequency.
Since surrounding noises cause vibrations of the LASER
mechanical structure, which changes the output
frequency, a compensation controller is used to reduce
the error signal noise. The FxLMS algorithm is used as
the control algorithm.
A very fast DSP-based controller, enables real time
compensation for the acoustic interference by
changing the external cavity length. The DSP changes
the voltage applied to the piezoelectric actuator
according to the FxLMS adaptive filter algorithm.
NOISE
P(z)
Measured in reality
Expected from theory
AFEKA Tel-Aviv Collage of Engineering
Moskovitch Moran & Fishelzon David
052-2725075
052-8735585
Email:
[email protected]
[email protected]
Web:
Laser4Bec.Blogspot.com
The Ficus Tree Advanced DSP Lab Web site :
http://highlearn.afeka.ac.il/users/www/9357/index.html
The simulated system output :
dn
en
W(z)
xn
yn
S(z)
Acknowledgements
y’
n
S’(z)
x’
LMS
Output Noise
Input Noise
n
The controller’s job is to pick up the acoustic noise and compensate for this noise by
changing the piezoelectric actuator's voltage. The S’(z) system is used to enable
convergence of the adaptive filter despite the non-minimal phase system of the piezo.
The FxLMS algorithm worked well and achieved significant Noise reduction.
The mean square error of the output signal was reduced by the DSP running the
control algorithm
We would like to thank the BEC laboratory people who
helped us with measurements on the real system
A special thanks to our instructor in the Advanced DSP
Lab, Mr. Alon Slapak who helped us find this algorithm
and implement it.