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Building and Testing a
Balance Detector
David Grayson – University of Illinois at Urbana-Champaign – [email protected]
Prof. John Howell – University of Rochester
Research Experience in Physics and Astronomy for Undergraduates at the University of Rochester
Balance detectors measure laser
beam intensity difference
Input Power
Intensity I2
-15 V
0V
Laser beams (same color)
+15 V
Intensity I1
Output: Voltage propotional to (I1-I2)
(up to ±15V)
Voltage is 0 when I1=I2
2
Balance detectors are useful
• Prof. Howell’s lab: A
novel new experiment
to test general
relativity on the
tabletop
• David Starling
• Manuel Alves
• Prof. Howell’s dream
Manny
3
Our balance detectors are simple
Original detector built
by Michael Pack
Circuit diagram obtained
by examination.
4
I built two balance detectors
BD1
BD2
5
I tested the balance detectors
6
The balance detectors were
sensitive enough
BD2 Sensitivity Test
BD1 Sensitivity Test
20
10
10
5
0
-20
0
20
40
60
80
Output Voltage (V)
Output Voltage (V)
15
5
0
-40
-20
0
20
40
-5
-5
-10
Laser Beam Intensity Difference (µW)
Sensitivity = Slope = .27 V/μW
-10
Laser Beam Intensity Difference (uW)
Sensitivity = Slope = .24 V/μW
7
Response near zero
output voltage
sensitivity
intensity
intensity
V  k ( I1  I 2 )
V = k(I1 – I2)
Procedure:
Too hard
1. Adjust the system to make I1 = I2
2. Measure V
1. Adjust the system to make V = 0
2. Measure I1-I2
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The balance detectors’ response
near zero was good
BD2 Response Near Zero
BD1 Response Near Zero
Sample points where output voltage is zero
10
5
0
0
100
200
300
400
500
-5
-10
Intensity Difference (µW)
Intensity Difference (µW)
Sample points where output voltage is zero
15
10
5
0
0
100
200
300
400
500
-5
-15
Average Intensity (µW)
Result: Imbalances of 1 μW are
detected in the 15-200 μW range.
Average Intensity (µW)
Result: Imbalances of 1 μW are
detected in the 15-200 μW range.
9
Now the balance detectors are
being used in the experiment.
10
Acknowledgments
• My Advisor: Professor John Howell
• Program Coordinator: Connie Jones
• The Research Experience in Physics and
Astronomy for Undergraduates at the University
of Rochester, funded by National Science
Foundation Grant No. PHY-0552695.
• John Gresty
• David Starling
• Ben Dixon
Questions? Contact David Grayson, [email protected]
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Here be dragons
12
I made a quick prototype
• Used only equipment
that I found in the lab
13
The prototype was sensitive
enough
Prototype Sensitivity Test
10
Output Voltage (V)
5
0
-60
-40
-20
0
20
40
-5
-10
-15
Laser Beam Intensity Difference (μW)
Sensitivity = Slope = .26 V/μW
14