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Condition Air Pressure: 1 atm Relative angle between B and E: ~90 deg

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Condition Air Pressure: 1 atm Relative angle between B and E: ~90 deg - PowerPoint PPT Presentation

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Ellipsometry noise spectrum, suspension t ransfer function measurement and closed-loop control of the suspension system in the Q & A experiment. Injection Optical Bench. Suspended FPI & Rotating Magnet. Detection Optical Bench. light intensity. QPD3. Photodiode. Correction Signal.

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Ellipsometry noise spectrum, suspension transfer function measurement and closed-loop control of the suspension system in the Q & A experiment

Injection Optical Bench

Suspended FPI & Rotating Magnet

Detection Optical Bench

light intensity



Correction Signal

Rotating B field

220 Hz

Longitudinal Control

Beam Centering Control

EOM electro-optical modulator; λ/2 WP half-wave plate; l/4 WP quarter-wave plate; L1, L2, L3 lenses; M1, M2, M3 mirrors; Mpzt piezo-driven mirror; PBS polarizing beam splitter; BS beam splitter; FR Faraday rotator; CM1, CM2 cavity mirrors; B magnetic field; PD1, PD2, PD3 photo-detector; QPD1, QPD2 quadrant photo-detector; FC Faraday cell; DAQ data acquisition system; LA lock-in amplifier.

yaw alignment on

yaw alignment off

Alignment Control (Yaw)


220 kHz







l/2 WP

Fabry-Perot Cavity



Effect of alignment control







Peaks were suppressed







Alignment off



12 MHz

Alignment on

Closed-loop residual cavity length noise




But for frequency above 15 Hz, the noise density is higher

We are still working on this…

Sheng-Jui Chen, Hsien-Hao Mei and Wei-Tou Ni

  • Abstract

Alignment error signal spectrum

The Q & A experiment, aiming at the detection of vacuum birefringence predicted by quantum electrodynamics, mainly consists of a suspended 3.5 m Fabry-Perot cavity, a rotating permanent dipole magnet and an ellipsometer. The 2.3 T magnet can rotate up to 10 rev-per-sec, introducing an ellipticity signal at twice the rotation frequency. The X-pendulum gives a good isolation ratio for seismic noise above its main resonant frequency, in our case 0.3 Hz. At present, the ellipsometry noise decreases with frequency, from 110-5 rad·Hz-1/2 at 5 Hz, 210-6 rad·Hz-1/2 at 20 Hz to 510-7 rad·Hz-1/2 at 40 Hz. The shape of noise spectrum indicates possible improvementwhen the movement between the cavity mirrors is further reduced. From the preliminary result of yaw motion alignment control, it can be seen that some peaks due to yaw motion of the cavity mirror was suppressed. In this paper, we first give a schematic view of the Q & A experiment, and then present the measurement of transfer function of the compoundX-pendulum-double pendulum suspension. A closed-loop control was carried out to verify the validity of the measured transfer function. The ellipsometry noise spectra with and without yaw alignment control and the newest improvement will be presented.

Control parameters need to be optimized

A closed-loop control was carried out to verify the validity of measured transfer functions:

  • Preliminary results



The noise floor for ellipticity detection decreases with frequency and reached 310-7 radHz-0.5 at 75 Hz. The noise floor seems to have a good correlation with the cavity mirror’s motion which can be seen more clearly in measurements in which alignment control was turned on. Some peaks were suppressed and noise floor below 15 Hz was lowered. But for noise above 15 Hz, the noise floor is higher than that without alignment control. That is probably due to the spurious alignment error signal coming from common beam spot motion and higher-order Gaussian mode.

  • Vacuum Birefringence

4.05 μm  0.23 μm

Predicted by QED, vacuum is birefringent under the influence of a strong magnetic field, for B=2.5 T:

Sensitivity curve of ellipticity detection (Alignment off)


Air Pressure: 1 atm

Relative angle between B and E: ~90 deg

15.48 μrad  4.4 μrad

And induced ellipticity:

The noise density decreases with frequency

  • Feedback control

610-6 radHz-0.5

We use Pound-Drever-Hall technique for longitudinal control, a 2-loop control including laser frequency control and cavity length control, and differential wave-front sensing technique for alignment control:

210-6 radHz-0.5

510-7 radHz-0.5

310-7 radHz-0.5

Detection band

  • Experiment setup

To be added, not yet implemented

Effect of alignment control

After passing through the optics necessary for Pound-Drever-Hall error signal extraction, the light is polarized by a high extinction-ratio polarizer and sent into the Fabry-Perot cavity with a 2.3 T rotating magnetic field in its resonant path. After coming out from the CM2, the polarization of the light becomes elliptically polarized. Before analyzed by another high extinction-ratio polarizer, this ellipticity is transformed into polarization rotation and modulated by a Faraday cell for lock-in detection.

This result is limited by the laser frequency noise

  • Transfer function measurement of the double pendulum

Alignment off

In Q & A experiment, the cavity mirrors are suspended by the X-pendulum and a double pendulum for seismic noise isolation. The X-pendulum, designed and developed by TAMA laser-interferometric gravitational wave detection team, has a fundamental resonant frequency of 0.3 Hz in our experiment and provides a good isolation ratio in our detection band, 10~20 Hz. The double pendulum is used for active control of cavity length, mainly for compensating the large resonant displacement of the X-pendulum. For this purpose, we measured the transfer functions of double pendulum in two degrees of freedom:

Preliminary result of alignment control

  • Future works
  • Add alignment-control to rest degrees of freedom
  • Maintaining FP cavity at its optimum working point
  • Stabilize laser frequency to an external fixed cavity (L=17 cm, measured finesse~45000)
  • A better frequency standard for cavity length below 220 Hz
  • Add a fiber as a simple mode cleaner

Reducing alignment signal from higher-order Gaussian mode

  • Goal sensitivity 510-8 radHz-0.5 at 10Hz ~ 20 Hz

Amaldi6, June 20-24, 2005 Okinawa, Japan