Virgo alignment system overview
● Linear Alignment principle
● Optical configuration
● Present situation
● Foreseen developments
2 LA electronics meeting, Cascina, 25.01.2006
The VIRGO Interferometer
N W
EOM
Task of the alignment system:
Keep 6 mirrors and input beam aligned
5 output beams for obtaining error signals
Basics of error signal retrieval
Phase modulation of input beam
Demodulation of photodiode signals at different output beams
=> longitudinal error signals
Quadrant diodes in output beams
=> Alignment information (differential wavefront sensing)
Anderson-Giordano technique
2 quadrant diodes after arm cavities
4 LA electronics meeting, Cascina, 25.01.2006
Can have 1 normal diode and 2 quadrant diodes at each output port
Detection
Linear alignment setup
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Control noise
Present Virgo noise budget
Optimized alignment noise budget
Maximized power
Optimized mirror centering (0.2 mm)
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Present situation
Frascati group is leaving Virgo
Since 01/2006
Frascati’s responsibilities
Original design of alignment system
Strategy, optics, prototype experiments, …
Design & realization of electronics
Problem
Continue support for alignment electronics Make new modules / spare modules
Continue development for new requirements
Developments
Present developments
More modules needed
Installation of 9th quadrant diode (maybe 10th) Spares needed
New Annecy local oscillator boards, compatible with alignment
Phase shifters for standard photodiodes
Possible developments
Substitute Si diodes with InGaAs diodes
Better quantum efficiency Lower bias voltage
=> higher power capability
lower noise
Reduction of electronics noise
Better preamplifier: 5 pA/rtHz -> 1.6 pA/rtHz (?) DC signals: pre-amplification / pre-shaping
Fast quadrant centering system
(Napoli is working on that)
LA noise limits sensibility (especially at low frequ.)
End
Quadrant photodiode
•type EG&G YAG 444
•sensitivity = 0.45 A/W
•DC power = 3 mW
max•transimpedance = 2 k
•Bias voltage = 180 V
12 LA electronics meeting, Cascina, 25.01.2006
QD electronics
demodulator phase shifter
Quadrant diode box
Scheme of LA electronics
ADC noise Preamp. noise
Shot noise
Low-pass filter AC: Gain 200
DC: Gain 1 diff. sig.
non-diff.sig.
Non-optimal treatment of DC signals QD box VME
14 LA electronics meeting, Cascina, 25.01.2006
Noise measurements
after demodulator
quadrant diode AC output
Theoretical shot noise M. Mantovani
demodulator gain
C7 matrix
ThetaX
PR BS NI NE WI WE B2_q1_DC 2.5 -4.5 -6.4 15 B5_q1_DC 10
B2_q1_ACp -24 9.41 17.9 5.38 B1p_q1_ACp 0.052 -0.108 0.0375 1 0.0729 B7_q1_ACq 0.042 -0.035 0.375 -0.027 B7_q2_ACq 0.042 -0.035 0.375 -0.027 B8_q1_ACp -0.25 0.65 0.052 0.071 B8_q1_ACq -0.25 -0.65 -0.052 -0.071 ThetaY
PR BS NI NE WI WE B2_q1_DC 1 B5_q1_DC 10
B1p_q1_ACq 1 B7_q1_ACp 0.33 0.36
B7_q2_ACp 0.68 0.36 B7_q2_ACq -0.36 -0.36 B8_q1_ACp -0.46 -1 -0.36 B8_q2_ACp 1
16 LA electronics meeting, Cascina, 25.01.2006
Electronics noise vs. C7 noise (NE ty)
Total LA electronics noise (C7, calculated)
WE ty pure DC error signal => no extra noise vs. AC WI under local control
Simulated WI LA curves for comparison C7 noise not limited by LA electronics noise
(but: excess noise)
18 LA electronics meeting, Cascina, 25.01.2006
Influence of QD power
Can gain ~ factor 10
by increasing QD powers
