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Quadrant diode configuration

Quadrant diode configuration. ‘X’ or ‘+’ configuration? ISYS quadrant diodes. ‘X’ or ‘+’ configuration?. Comparison ‘X’ and ‘+’ configuration. Relative sensitivity of X configuration Virgo QD’s are used in X configuration Same signal-to-noise-ratio as + config. Couplings

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Quadrant diode configuration

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  1. Quadrant diode configuration • ‘X’ or ‘+’ configuration? • ISYS quadrant diodes

  2. ‘X’ or ‘+’ configuration?

  3. Comparison ‘X’ and ‘+’ configuration • Relative sensitivity of X configuration • Virgo QD’s are used in X configuration • Same signal-to-noise-ratio as + config. • Couplings • Horizontal/vertical error signal couplings • X config: couplings for h+v miscentering • Misalignment sensitivity • Strong horizontal or vertical misalignment: • X config may give no error signal • Conclusion • For new QD front ends • “+” configuration desirable vs.

  4. Simulation h/v couplings • Simulation of horizontal/vertical coupling • Normalized asymmetry fluctuates − 0.5 … + 0.5 (C7) • ‘X’ config: 4 % coupling Θx − Θy ‘+’ config: none Q81 (8 hours) dh = dv = 0.3 Normalized asymmetry Power as seen by QD (horizontal) C7 Normalized asymmetry

  5. ISYS quadrant diodes • Overview

  6. ISYS Quadrants PHD (IMC, RFC) (Preliminary results) Quadrant photodiode specifications - Centronic QD40-4x, silicon photodiode (EG&G YAG 444-4) - 7.98mm diam, 200microns gap, 0.4 A/W (11.4 mm, 0.45 A/W) - 120V bias (IMC & RFC) (180 V) - 10mA peak current (3 mA) - “+” geometrical configuration (“X” configuration) Quadrant “box” specs. - 6 MHz & 22 MHz, demodulation done onboard (RFC, IMC respectively) - IMC & RFC: involved in servo-loops - vertic/horiz error signals (AC & DC) provided onboard, BNC connectors (unipolar) - R_ac: 50 kohm - 4quadrants, shot noise limited for 16 mA, should be 0.6mA, (need TBC and check reason for extra noise) (in that case, S/N would be equivalent to Frascati quadrants) Frascati QD’s

  7. ISYS Quadrants PHD (IMC, RFC, PMC, SL) Centering electronics

  8. ISYS Quadrant PHD (IMC, RFC, PMC, SL) • Discussion • we can work shot noise limited with present ISYS quadrants • Question : • Is it worth to extend LA standard for ISYS quadrants (2*4)? • Main advantage: standardization • Price to pay: • - Extra demodulation boards • - Extra cabling • - Swap translation stages • - rearrange optical setup? (VIRGO quadrant box larger) • What about QDs for monitoring SL & PMC (nearly same as IMC & RFC) (14 MHz)? • What about DC QDs (BMS, MC end mirror transmission)? • at present: EGO is to provide those QDs (UDT photodiodes already bought), • use temporarily PMC QDs (for BMS), and Nice QD for MC mirror

  9. End

  10. LA electronics TStage driver Demodulator board (2 channel) Phase shifter (2 channel) QD front end

  11. Quadrant photodiode • type EG&G YAG 444 • sensitivity = 0.45 A/W • DC power = 3 mWmax • transimpedance = 2 k • Bias voltage = 180 V

  12. QD electronics phase shifter demodulator Quadrant diode box

  13. Scheme of LA electronics Low-pass filter AC: Gain 200 diff. sig. QD box Shot noise VME non-diff.sig. DC: Gain 1 Preamp. noise ADC noise Non-optimal treatment of DC signals dominated by ADC noise (but were not foreseen as error signals)

  14. Noise measurements after demodulator demodulator gain quadrant diode AC output Theoretical shot noise M. Mantovani

  15. Influence of QD power Can gain ~ factor 10 by increasing QD powers

  16. Fast quadrant centering system • Couplings • Coupling with longitudinal error signals • Horizontal/vertical error signals are coupled if h & v miscentering • Coupling with waist mismatch • Coupling with longitudinal noise • Better rms centering required • See G. Vajente’s talk (Napoli 03/10/2005) • Bad mirror motion reconstruction • Not understood • Fast QD centering system could remain ON all time • Control signal would be used for beam stabilization • Present system too noisy

  17. Requirements for centering With the present centering configuration (activates every few minutes) RMS change of a factor of 4 Extrapolating from these measurements… Taking a reasonable range around the “minimum”, the maximum DC asymmetry must be less than 0.1 Even with the best centering rate now possible (every 4-5 seconds) we can’t reach this precision.

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