Virgo alignment system overview

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Virgo alignment systemoverview
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The VIRGO Interferometer
Task of the alignment system Keep 6 mirrors and
input beam aligned 5 output beams for obtaining
error signals
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Basics of error signal retrieval

• Phase modulation of input beam
• Demodulation of photodiode signals at different
  output beams
• gt longitudinal error signals
• Quadrant diodes in output beams
• gt Alignment information
• (differential wavefront sensing)
• Anderson-Giordano technique
• 2 quadrant diodes after arm cavities

Quadrant diode
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Detection
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Linear alignment setup
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Present Virgo noise budget
Control noise
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Optimized alignment noise budget
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Present situation

• Frascati group is leaving Virgo
• Since 01/2006
• Frascatis 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

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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
• gt higher power capability
• lower noise
• Reduction of electronics noise
• Better preamplifier 5 pA/rtHz -gt 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.)

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End
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Quadrant photodiode

• type EGG YAG 444
• sensitivity 0.45 A/W
• DC power 3 mWmax
• transimpedance 2 k?
• Bias voltage 180 V

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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
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Electronics noise vs. C7 noise (NE ty)
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Total LA electronics noise (C7, calculated)
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