High mechanical Q-factor measurement of Si using a 3D cantilever support

Thermal noise in test mass substrates and coatings is a significant noise contribution in the detection band of current and proposed future gravitational wave detectors. Substrate thermal noise can be reduced by using high mechanical Q-factor materials and cooling the test mass mirrors. Silicon is a promising potential candidate for the next generation detector test masses. The low thermal expansion and high thermal conductivity of silicon allow efficient cryogenic operation, and a significant increase in the amount of optical power that can be used in the detectors by decreasing thermal deformation and aberration. Mechanical stress, damage, poor surface quality or contamination can result in increased loss and thermal noise. Therefore, the characterization of mechanical loss in silicon test masses is necessary. In this project, we developed a technique to measure high Q-factor mechanical modes. We used finite element modeling to optimize the design of the test mass support structure to minimize the loss coupling from the support structure over a wide frequency range. Mechanical Q-factors of the order of 10⁷ were achieved for several modes of a 10 cm diam. × 3 cm cylindrical silicon test mass with such a support at room temperature.

A multi-orientation low-frequency rotational accelerometer

We present a low frequency rotational accelerometer coined ALFRA with a few nrad/Hz readout sensitivity above 20 mHz and 0.1 nrad/Hz above 50 mHz. The ALFRA is a beam-balance style rotation sensor, which pivots about a cross flexure designed to allow mounting with any orientation, the axis of the pivot determining which rotational component is measured. The high sensitivity is achieved through the use of a walk-off sensor readout used in a feedback loop with an electromagnetic coil to keep the beam dynamically locked. The ALFRA is relatively compact for a ground rotation sensor, measuring at 780 × 240 × 55 mm³.

Six degrees of freedom vibration isolation with Euler springs

A novel design allows column springs in Euler buckling mode to be laterally stable and thus provides vibration isolation in six degrees of freedom. Analytical models of the stiffness were used to develop a design with a vertical resonance of 1.13 Hz, a horizontal resonance of 1.68 Hz, and a roll resonance of 2.58 Hz. A prototype vibration isolator reduces vertical vibration by a factor of 2 at 2 Hz. Vertical, horizontal, and roll vibrations are reduced by a factor of 100 at frequencies above 20 Hz.

A laser walk-off sensor for high-precision low-frequency rotation measurements

We present an optical walk-off sensor with an angular sensitivity of a few nrad/Hz above 1 mHz and 0.4 nrad/Hz above 100 mHz. This experiment furthers previous research into the walk-off sensor capabilities through an improved input laser, reduction in air optical travel length, and position control on photo-diodes. The angle change measured in this walk-off scheme features a knife edge to split the beam into two separate fiber coupled photo-diodes to minimize power dissipation in the thermally sensitive region. Using this photo-diode power differential as an error signal, a simple control scheme is used to maintain the balance position, increasing common mode rejection and improving dynamic range by mitigating thermal drift. The in-vacuum component of the optical readout takes up a volume less than 100 mm × 100 mm × 50 mm. This experiment shows that the walk-off sensor provides a simple and compact readout scheme with nanoradian sensitivity for angle sensing at low frequencies.