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OSEM Development UK Advanced LIGO project

OSEM Development UK Advanced LIGO project. Stuart Aston University of Birmingham for the UK Advanced LIGO Team LSC Livingston, March 2004 LIGO-G040064-00-K. Overview. Requirements: Generic OSEM Requirements OSEM Noise Performance Requirements OSEM Development Concepts:

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OSEM Development UK Advanced LIGO project

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  1. OSEM Development UK Advanced LIGO project Stuart Aston University of Birmingham for the UK Advanced LIGO Team LSC Livingston, March 2004 LIGO-G040064-00-K LIGO-G040064-00-K

  2. Overview • Requirements: • Generic OSEM Requirements • OSEM Noise Performance Requirements • OSEM Development Concepts: • Geometric based OSEMs • Interferometer based OSEM • Performance Summary • Conclusions: • OSEM Recommendations UK Advanced LIGO Team LIGO-G040064-00-K

  3. Generic OSEM Requirements • Range: • 3mm (peak-peak) working range (1.06mm rms), incorporating: •  1mm operating range •  ½mm tolerance for relative positioning of OSEM and sensed mass • Fit: (sensors must fit actuator) • Sense motion of mass through bore hole of coil former • Constraints (for the case of the TM quad) restrict layout envelope to 40mm (diameter) x 70mm (length), including connectors and mounts • Vacuum Compatibility: • Sensor components must meet the vacuum compatibility requirements appropriate to an Advanced LIGO vacuum chamber • Electrical Compatibility: • Sensor drive and output must be compatible with the specification of cables passing through the SEI system (ideally using LIGO cables) UK Advanced LIGO Team LIGO-G040064-00-K

  4. OSEM Noise Performance Requirements • Sensitivity Requirements for Local Control OSEMs: • Assume Eddy current damping and ISC feedback, most stringent requirement is MC longitudinal = 1.72x10-11m/Hz * • For more detail, see E040108-00-K • Sensitivity Requirements for Global Control OSEMs: • Science mode noise requirement about 10Hz:  1x10-10m/Hz * • This figure also encompasses: • Transverse horizontal sensors on all suspensions • Other, less critical, degrees of freedom on RM and MC • Note that almost half the OSEMs fit one or other of these categories * Figures extracted from “Recommendation of a design for the OSEM sensors” document LIGO-E040108-00-K UK Advanced LIGO Team LIGO-G040064-00-K

  5. OSEM Concept Development • Geometric Sensors: • Prototype developed by C.C. Speake and S.M. Aston at the University of Birmingham • Parallel independent development undertaken by our collaborator N.A. Lockerbie at the University of Strathclyde • Basic Operation: • Use a reasonably collimated and strongly modulated light source • High power IRLED (880nm) emitter (OP-50L) • Beam passes through optical system which is partly mounted on an extension of the sensed mass • Beam is incident on a split photodiode detector • Detected photocurrents are demodulated to produce sum and difference signals: • Difference output provides useful displacement measurement read-out • Sum signal enables active intensity stabilisation of emitter output UK Advanced LIGO Team LIGO-G040064-00-K

  6. OSEM Concept Development • Geometric Sensor Prototype (University of Birmingham): Translation (± x) Photodiode IRLED OD-50L Cylindrical Lens Translation (± y) 10mm 41mm (25mm) Rotation (z) UK Advanced LIGO Team LIGO-G040064-00-K

  7. (BPW34S) (of OD-50L) OSEM Concept Development • Geometric Sensor Prototype (University of Strathclyde): UK Advanced LIGO Team LIGO-G040064-00-K

  8. OSEM Concept Development • Interferometric Sensor: • Development by C. Speake and S. Aston at the University of Birmingham • Basic Operation: • Use a collimated laser diode light source • Low power (3mW) emitter (HL6314MG) at 635nm • 3 output interferometer has single element “bug-eye” pd’s at each output • Polarising beam-splitter gives quadrature outputs for 2 ports • 3rd port output is 1800 out of phase (allows subtraction of dc offsets) • 3 outputs are subtracted in pairs (via 2 difference amplifiers) resulting in 2 dc corrected channels to be read-out • Signal processing allows the fringes to be counted as well as fractional fringe measurements to be made • Dynamic range limited by the coherence length of the laser diode UK Advanced LIGO Team LIGO-G040064-00-K

  9. 70mm QWP HWP QWP BS Adjustable Mirror PBS Attenuator and aperture QWP PD2 PBS PD3 LD PD1 Laser Diode and Collimation Optics Suspension Mirror 40mm Cat’s Eye Polariser Quarter-wave plate Non-polarising beam-splitter Polarising beam-splitter Half-wave plate Mirror D A B C E G F Pd3 Pd1 Pd2 OSEM Concept Development • Interferometric Sensor Prototype (University of Birmingham): UK Advanced LIGO Team LIGO-G040064-00-K

  10. OSEM Development Performance • Geometric Sensors: • Shot noise limited performance of  1.5x10-11m/Hz about 10Hz and essentially flat down to 1Hz. • But this result was not achievable over the whole working range! • Common suspect component of excess noise is the IRLED emitter • Significant risk that there are no other alternative high power, low noise and good beam quality devices available • Interferometric Sensor: • Performance of  5x10-13m/Hz about 10Hz is achievable without a high power source or any modulation scheme • Perceived risk in complexity of construction and laser diode noise performance and MTBF (device screening maybe required) UK Advanced LIGO Team LIGO-G040064-00-K

  11. Conclusions • OSEM Recommendations: • We recommend the use of the interferometric design wherever high sensitivity is required together with basic (modified) initial LIGO OSEMs for global control and certain other locations • Summary of Rationale: • Interferometric concept is the most secure approach to reaching the sensitivity requirement • Higher component cost partly offset by possible elimination of eddy current dampers UK Advanced LIGO Team LIGO-G040064-00-K

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