1 LA electronics meeting, Cascina, 25.01.2006

Virgo alignment systemoverview

● 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

3 LA electronics meeting, Cascina, 25.01.2006

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 technique2 quadrant diodes after arm cavities

Quadrant diode

4 LA electronics meeting, Cascina, 25.01.2006

Can have 1 normal diode and 2 quadrant diodes at each output port

Detection

5 LA electronics meeting, Cascina, 25.01.2006

Linear alignment setup

6 LA electronics meeting, Cascina, 25.01.2006

Control noise

Present Virgo noise budget

7 LA electronics meeting, Cascina, 25.01.2006

Optimized alignment noise budget

Maximized powerOptimized mirror centering

(0.2 mm)

8 LA electronics meeting, Cascina, 25.01.2006

Present situation

Frascati group is leaving VirgoSince 01/2006

Frascati’s responsibilitiesOriginal design of alignment system

Strategy, optics, prototype experiments, …

Design & realization of electronics

ProblemContinue support for alignment electronicsMake new modules / spare modulesContinue development for new requirements

9 LA electronics meeting, Cascina, 25.01.2006

Developments

Present developmentsMore modules needed

Installation of 9th quadrant diode (maybe 10th)Spares needed

New Annecy local oscillator boards, compatible with alignmentPhase shifters for standard photodiodes

Possible developmentsSubstitute Si diodes with InGaAs diodes

Better quantum efficiencyLower bias voltage

=> higher power capability lower noise

Reduction of electronics noiseBetter 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

11 LA electronics meeting, Cascina, 25.01.2006

Quadrant photodiode

•type EG&G YAG 444

•sensitivity = 0.45 A/W

•DC power = 3 mWmax

•transimpedance = 2 k•Bias voltage = 180 V

12 LA electronics meeting, Cascina, 25.01.2006

QD electronics

dem

odul

ator

phas

e sh

ifte

r

Quadrant diode box

13 LA electronics meeting, Cascina, 25.01.2006

Scheme of LA electronics

ADC noisePreamp. noise

Shot noise

Low-pass filter

AC: Gain 200

DC: Gain 1

diff. sig.

non-diff.sig.

Non-optimal treatment of DC signalsdominated by ADC noise(but were not foreseen as error signals)

VMEQD box

14 LA electronics meeting, Cascina, 25.01.2006

Noise measurements

after demodulator

quadrant diode AC output

Theoretical shot noise M. Mantovani

dem

odul

ator

gai

n

15 LA electronics meeting, Cascina, 25.01.2006

C7 matrix

ThetaX PR BS NI NE WI WEB2_q1_DC 2.5 -4.5 -6.4 15B5_q1_DC 10B2_q1_ACp -24 9.41 17.9 5.38B1p_q1_ACp 0.052 -0.108 0.0375 1 0.0729B7_q1_ACq 0.042 -0.035 0.375 -0.027B7_q2_ACq 0.042 -0.035 0.375 -0.027B8_q1_ACp -0.25 0.65 0.052 0.071B8_q1_ACq -0.25 -0.65 -0.052 -0.071

ThetaY PR BS NI NE WI WEB2_q1_DC 1B5_q1_DC 10B1p_q1_ACq 1B7_q1_ACp 0.33 0.36B7_q2_ACp 0.68 0.36B7_q2_ACq -0.36 -0.36B8_q1_ACp -0.46 -1 -0.36B8_q2_ACp 1

ThetaX has undergone second diagonalization => mixing of all signals

16 LA electronics meeting, Cascina, 25.01.2006

Electronics noise vs. C7 noise (NE ty)

17 LA electronics meeting, Cascina, 25.01.2006

Total LA electronics noise (C7, calculated)

WE ty pure DC error signal => no extra noise vs. AC

WI under local controlSimulated WI LA curves for comparison

C7 noise not limited byLA electronics noise

(but: excess noise)

18 LA electronics meeting, Cascina, 25.01.2006

Influence of QD power

Can gain ~ factor 10by increasing QD powers