1 LA electronics meeting, Cascina, Virgo alignment system overview ● Linear Alignment principle ● Optical configuration ● Present situation ● Foreseen developments

2 LA electronics meeting, Cascina, 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, 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 Quadrant diode

4 LA electronics meeting, Cascina, Can have 1 normal diode and 2 quadrant diodes at each output port Detection

5 LA electronics meeting, Cascina, Linear alignment setup

6 LA electronics meeting, Cascina, Control noise Present Virgo noise budget

7 LA electronics meeting, Cascina, Optimized alignment noise budget Maximized power Optimized mirror centering (0.2 mm)

8 LA electronics meeting, Cascina, 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

9 LA electronics meeting, Cascina, Developments Present developments More modules needed Installation of 9 th quadrant diode (maybe 10 th ) 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

11 LA electronics meeting, Cascina, 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, QD electronics demodulator phase shifter Quadrant diode box

13 LA electronics meeting, Cascina, 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 dominated by ADC noise (but were not foreseen as error signals) VME QD box

14 LA electronics meeting, Cascina, Noise measurements after demodulator quadrant diode AC output Theoretical shot noise M. Mantovani demodulator gain

15 LA electronics meeting, Cascina, C7 matrix ThetaX PR BS NI NE WI WE B2_q1_DC B5_q1_DC 10 B2_q1_ACp B1p_q1_ACp B7_q1_ACq B7_q2_ACq B8_q1_ACp B8_q1_ACq ThetaY PR BS NI NE WI WE B2_q1_DC 1 B5_q1_DC 10 B1p_q1_ACq 1 B7_q1_ACp B7_q2_ACp B7_q2_ACq B8_q1_ACp B8_q2_ACp 1 ThetaX has undergone second diagonalization => mixing of all signals

16 LA electronics meeting, Cascina, Electronics noise vs. C7 noise (NE ty)

17 LA electronics meeting, Cascina, 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, Influence of QD power Can gain ~ factor 10 by increasing QD powers