Identifying and eliminating limiting noise sources of GEO600

1. ILIAS WG1Meeting, Hannover, September `04 Joshua Smith for the GEO team Identifying and eliminating limiting noise sources of GEO600

2. P and Q • TFs depend on demod. phase, now P is close to optimal for diff. arm signals at DC • GW content present in both: Q = h + nQ P = h + nP

3. Sept ‘04 vs. Theoretical

4. Noise Projections to hP

5. Noise Projections hQ

6. Oscillator Noises • PR & SR oscillator noise couplings to be determined • MID oscillator noises: • amplitude:2-3 below from 400Hz to 1.1kHz (at least)! • amplitude stab. loop (compare RF, stable DC ref) • first tests: no change in sensitivity • Phase: 8 below @ 1kHz • projection TBD next week • crystal oscillators (-165 dBc/rt(Hz) possible) • `2f’ local oscillator

7. Dark noise + Shot noise • HPD dark noise measured with no light, may underestimate level • Shot noise (not shown, but at same level) calculated from current of HPD in lock, also slight underestimate • Monday: measure while MI locked on another diode (alignment quadrant cam), demodulate HPD signal at Fmi + 100kHz or so

8. Modulation Depths Scanning FP measurement of light reflected from MPR C SR SR Normalized: Carrier: 1 SR-SB: 0.068 MI-SB: 0.020 PR-SB: 0.027 PR PR MI MI

9. SR Mod. Depth • SR loop noise was not projected to be limiting • Possible explanation: coupling via large beats on Common-mode diode, or amplitude noise at dark port?

10. PR mod depth • Dark noise of Quadrant diode drives frequency noise via loop gain • Switch to single-element diode allows (factor 2) reduction of mod depth without adding noise

11. Other Bad Guys • acoustic & air current noises • Couples at some level, output bench and MIC breadboard strongest • Output telescope, HPD in vacuum now (some improvement@100Hz, cause?) • perhaps enclosure for MIC components • scattered light • visible problem, nonlinear, excited with e.g. stomps • Removed some clipping by shifting MSRr suspension • frequency noise • lower noise on main diode, improved gain distribution • complex loop not yet fully understood

12. Dark Port Amplitude Noise Contrast Defect = P_carr_darkp / P_carr_BS = 1.5mW / 270W = 5e-6 Normalized C: 1 MI L: 16.4 SR: 4.8 MI H: 3.9 • Apparently reduced by SR modulation depth • Coupling may not be linear w/ eg. mod depth due to different relative SB ratios RIN @ dark port = V/rt(Hz) / V_DC = 5e-7/rt(Hz) @ 100 Hz = 5e-8/rt(Hz) @ 1kHz MI SR MI C

13. Removing the Noise • MID fast path noise [factors are for that noise source, not h(t)] • bypass sqrt-circuit, reduces noise, increases nonlinearity • build noise reduction loop around HVA, factor 10 possible • Reduce Vbias in lock (controlled by LabView) • dark & shot noise • Increase laser power (2W to 10W into MC1) should give factor 2.2 • Increase PR factor by replacing T=1.35% MPR with T~0.1% mirror, circulating ~ 10kW rather than 1.4kW (270 W now), factor 2.7 • Laser amplitude noise • second loop commissioned, noise seems to be added after sensor, perhaps sensing at later point is required • identify the cause, coupling • Front end (signal) limited loops (SRC, MID < 200Hz) • Cross-projections : MID to SRC, etc. • coupled noise decrease with reduction of noise in other loops

14. ESD Nonlinearity • Dielectric polariz. ~ V and F on dielectric ~ V, thus F ~ V**2 • Without sqrt-circuit this creates beats and 2f up-conversion • beat ratio = <A>/A(beat) = offset*(A(1)+A(2))/(A(1)*A(2)) • 2f upcon ratio =A(f1)/A(f2) = 4*offset/A(f1) • sets a limit on dynamic range

15. Nonlinear ESD Noise

16. Goals • Improve sensitivity before S4 • Improve understanding of couplings • Understand noise in PR loop (by next mtng!)

17. MID loop (MI diff. arm-length) • LSC_MID_EP-P_HP • main GW signal, inphase • LSC_MID_FP-MCE-MCN  diff. FB sampled after MID electronics • LSC_MID_FP-MCE-HV, LSC_MID_FP-MCN-HV  diff. FB sampled after HVA

18. MI Fast Path