Suspension_grabeel

One of LIGO's optics suspensions installed in a large vacuum chamber. (Credit: G. Grabeel/LIGO)

Vibration Isolation

For an instrument that needs to remain as still as possible, it is ironic that LIGO is so sensitive that it can feel the smallest vibrations from near and far. LIGO is essentially a giant seismometer capable of sensing vibrations from traffic on nearby roads, weather patterns on the other side of the continent, staff biking alongside detector arms, ocean waves crashing on shores hundreds of miles away, and of course nearly every significant earthquake on the planet. Since gravitational waves will make themselves known through vibrations in LIGO's mirrors, the only way to make gravitational wave detection possible is to isolate LIGO's components from environmental vibrations to unprecedented levels. The change in distance between LIGO's mirrors (test masses) when a gravitational wave passes will be on the order of 10-19 m. To achieve this level of sensitivity, LIGO was constructed with multiple levels of active and passive vibration isolation systems. Many of LIGO's larger infrastructure systems that provide some additional levels of isolation are discussed in previous sections. But LIGO's most sensitive components (its optics) required even more complex and highly specialized mechanisms for isolating them from even the smallest imaginable vibrations.

Outside of its pre-stabilized laser, LIGO's vibration isolation systems are comprised of two basic elements: Optics Suspensions and Seismic Isolation.


Optics Suspensions (Passive Isolation)

LIGO's mirrors must be so well shielded from vibration that the random motion of the atoms within the mirrors and their housings can be detected. To achieve this level of shielding, each of LIGO's 40 kg test masses is suspended within a 360 kg quadruple pendulum system (or 'quad'). This system of multiple pendulums significantly reduces motion at the level of the test masses where it really counts. Suspensions make use of both passive and active vibration isolation components.

Vibration Isolation Labeled Quad with Side View

 

Two "chains" of suspended masses hang back to back in each suspension system: the main chain and the reaction chain (see the photo above). Each chain contains four masses. In the main chain, the top two are made of steel, and the bottom two of pure fused silica (in the reaction chain, the upper cylinder is made of metal). The lower-most cylinder in the main chain is the test-mass, measuring 34 cm x 20 cm (13.5 in. x 8 in.). Each 40 kg test mass is suspended by glass fibers 0.4 mm (400 microns) thick, which, since they do not expand or contract in response to temperature variations, isolate the mirrors against thermal noise. The total weight of the four masses is 120 kg.

Vibration Isolation Labeled Fibers

Close-up of fused silica glass fibers attached to one of LIGO's primary optics. The bottom of the photo shows the glass welds binding the fibers to the optic. The fibers taper to 0.4 mm. (Credit: Caltech/MIT/LIGO Lab)

 

To operate effectively, the lengths of LIGO's arm cavities (i.e. the distance between the test masses at the ends of each arm) must not vary by more than a fraction of a picometer (one-trillionth of a meter). To hold the masses in place, we need to push on them–but very carefully, and we use the reaction chain (see figure above) for that. Simple motors made of permanent magnets and electrical coils push on the upper masses; these 'voice coils' work like audio loudspeakers, with the coil producing a magnetic field which attracts or repels the magnets. On the test masses themselves, we use more gentle electrostatic forces, like that which attracts a balloon rubbed on a sweater to a wall (or hair to a comb on a dry day). The goal is to keep our hands off the masses as much as possible so they will move only due to gravitational waves.


Seismic Isolation (Active Isolation)

The first line of defense against vibration is LIGO's "active" damping system. Advanced LIGO's suspensions are mounted below in- and out-of-vacuum active vibration/seismic isolation systems, giving them the quietest possible environment for operation.

Internal seismic isolation platforms (ISIs) employ position- and vibration-sensors tuned to different frequencies of environmental vibrations, along with permanent-magnet actuators that work together to counteract ground movements keeping the instrument virtually motion-free. This level of isolation can reduce the magnitude of vibrations introduced to the suspensions (at the point of their attachment to the ISI) to a level of at most 2x10-13 m. The suspensions do the rest, reducing this noise level nearly six more orders-of-magnitude to achieve LIGO's desired detection sensitivity of 10-19 m.