About "aLIGO"

When reading or hearing news about LIGO, you may have wondered why we often refer to the instruments as “Advanced LIGO”, or “aLIGO”. Well, LIGO has actually been around for over 20 years, and our interferometers have undergone many changes and upgrades in that time. In fact, the first round of LIGO data collection, which took place between 2002 and 2010, used what we call “Initial LIGO”. As the name suggests, initial LIGO refers to the first interferometers that were built at the beginning of LIGO’s quest to detect gravitational waves.

When LIGO was approved for funding in 1990, it was understood that it would likely take decdes for the observatory to reach its full potential, with the instruments evolving over time. Though it was actively engaged in a search for gravitational waves, Initial LIGO was designed largely as a testbed or proof of concept instrument that would facilitate the development of new and better technologies that would be required to make LIGO, overall, achieve its full potential. 

Indeed, Initial LIGO operated for 9 years but never detected gravitational waves. But the lessons learned about how to operate, maintain, and improve one of the world’s most highly technological measuring devices, were incalculable. Construction of Advanced LIGO’s (aLIGO) upgraded components began in 2008, two years before iLIGO was retired.

Initial LIGO’s duties came to an end in 2010, at which point it was disassembled to make way for the new-and-improved Advanced LIGO detectors. Construction, preparation, installation, and testing of aLIGO took 7 years (from 2008 to 2015).

Ultimately, the goal is to make LIGO 10 times more sensitive than its initial instrument. Work continues on making this happen, and periodic stoppages in observing time-periods are already planned so further upgrades can be made in a systematic way. The image and table below illustrate the differences between Initial LIGO and Advanced LIGO.

iLIGO vs aLIGO (with caption)

 

Changed component

Initial LIGO

Advanced LIGO

Impact of the change

Mirrors (aka Test Masses)

25cm (9.8in) across

10cm (3.9in) thick

11kg (22lb)

34cm (13.4in) across

20cm (7.8in) thick

40kg (88lb)

In very basic terms, LIGO is designed to measure, to the highest level of precision possible, how far apart its mirrors ("test masses") are. We achieve this by using a laser. But as much as lasers are necessary, they are also problematic.

Laser photons striking the mirrors can move them resulting in a "recoil" effect. Unfortunately, any movement not caused by a gravitational wave is considered 'noise' in LIGO; noise that can drown out the signal from an actual gravitational wave. Thankfully, the principle of inertia helps reduce this problem: By increasing the size and mass of the mirrors, photon impacts are much less noticeable.

Lasers also heat the mirrors, which can change their shape, which in turn would affect the path of the laser and render our ability to detect gravitational waves impossible. Here again, ‘bigger is better’: a larger mirror can absorb more heat without deforming than a small mirror.

Suspension

Single pendulum

Quadruple pendulum

Initial LIGO’s test masses were suspended as a single-segment pendulum. Advanced LIGO’s test masses hang at the bottom of a 4-segment pendulum. LIGO exploits the basic principles of pendulums in that each link in the pendulum absorbs motion from above and prevents it from affecting the link below. This is so effective in aLIGO that any vibration at the top of the quad suspension is reduced by 100-million times by the time it reaches the bottom! This system is called a 'passive seismic isolation system', because the isolation happens because of the intrinsic properties of pendulums and the suspension's components.

Metal wires

Glass fibers

Initial LIGO used metal wires to hang the mirrors in their suspensions. Unfortunately, molecules in metal jiggle around a lot, introducing unwanted noise into the mirrors themselves. To reduce this problem, aLIGO's mirrors are suspended by glass/silica fibers since the molecules in silica are much less energetic than metal.

Seismic Isolation

Passive only

Passive + active

The term “seismic isolation” refers to the mechanisms designed to shield LIGO’s mirrors from physical vibrations caused by everything from trucks driving on nearby roads, to ocean movements, to earthquakes around the globe.

Initial LIGO used a ‘passive’ isolation system only; fancy shock absorbers that would absorb vibrations and prevent them from reaching the mirrors.

Advanced LIGO’s seismic isolation systems were greatly enhanced. In addition to the passive suspension system described above, aLIGO also employs “active” isolation systems. Here, sensors monitor movements occurring in hundreds of LIGO components and, in a process called 'feedback', send signals to ‘actuators’ that deliberately and with great precision counteract the detected movements. It's a process similar to that used by noise-canceling headphones.

The resulting combination of passive and active seismic isolation greatly reduces the vibrations reaching LIGO’s mirrors, making them 'feel' like they are floating in space. You can learn more about LIGO’s seismic isolation system here, and about LIGO’s feedback systems, here.