LIGO's Interferometer

Basic Michelson Labeled

Layout of a basic Michelson interferometer.

At their cores, LIGO's interferometers are Michelson Interferometers, the same sort of device that was invented in the 1880's:

  • They are L-shaped (not all interferometers are this shape)
  • Mirrors at the ends of the arms reflect light in order to create an interference pattern
  • A device measures the recombined light levels after the beams have been superimposed

But this is where the similarities end. The size and added complexity of LIGO's interferometers are far beyond anything Michelson could have envisioned or that his original interferometer could have achieved.

The first most obvious difference between a typical Michelson interferometer and LIGO's interferometers is its scale. LIGO's laser interferometers are by far the largest ever built. With arms 4 km (2.5 mi.) long, LIGO's interferometers are 360 times larger than the one used in the Michelson-Morley experiment (that interferometer had 11 m (33 ft.) long arms).

This is particularly important in the search for gravitational waves because the longer the arms of an interferometer, the farther the laser travels, and the more sensitive the instrument. Attempting to measure a change in arm length 10,000 times smaller than a proton means that LIGO has to be more sensitive than any scientific instrument ever before constructed.

Nevertheless, if LIGO's interferometers were simple Michelsons, even their 4 km long arms would not be long enough to detect gravitational waves on their own. But there are obvious practical limitations to building such a precision instrument much larger than 4 km.

Basic Michelson with FP labeled

Basic Michelson interferometer with Fabry Perot cavities. Additional mirrors are inserted near the beam splitter to facilitate multiple reflections of the laser, containing it within the interferometer and increasing the distance traveled by the beams. This greatly increases LIGO's sensitivity to the smallest changes in arm length.

This seeming paradox was resolved by altering the design of the Michelson to include a modification called "Fabry Perot cavities" (the figure at left shows how the basic Michelson design was modified to include Fabry Perot cavities).

The so-called Fabry Perot 'cavity' is actually the full 4 km length of each arm. Note the difference between this and the Michelson design. Here, additional mirrors inside each arm cause each laser beam to bounce back and forth along the full 4 km length of each arm about 400 times before it is merged with the other beam. All these reflections serve two functions:

1. It stores the laser light within the interferometer for a longer period of time, which increases LIGO's sensitivity

2. It essentially increases the distance traveled by each laser (and, in essence, the lengths of the arms) from 4 km to 1600 km!

Since we know that the longer the arms of an interferometer, the more sensitive the instrument is to vibration, this design significantly increases LIGO's sensitivity and enables it to detect changes in arm length much smaller than a proton--the size of changes expected to be caused by a gravitational wave.

There is an analogous effect in optical telescopes. Increasing the focal length of a telescope not only increases the magnification achievable by any given eyepiece, but it also amplifies the smallest vibrations in the telescope making the image jiggle wildly. In a telescope, these vibrations are wholly unwanted. LIGO, on the other hand, was designed to feel them. And at essentially 1600 km long, LIGO's interferometers can amplify the smallest conceivable vibrations enough that they are  detectable and measurable.

 

Length isn't the only limiting factor in LIGO's sensitivity; laser power is also a consideration. Just as increasing length increases the interferometer's sensitivity, increasing the laser power also enhances the instrument's performance, but in a different way. While increasing length magnifies smaller changes in arm length (one critical factor in detecting gravitational waves), increasing laser power results in increasing the interferometer's resolving power. In other words, the more laser light available to merge, the clearer the fringes that will appear in the photodetctor. There is another telescope analogy here. When you increase the aperture of the telescope, you collect more light, and the sharper the image becomes in the eyepiece.

Basic Michelson with FP and PR labeled

Basic Michelson with Fabry Perot cavities and Power Recycling mirror. LIGO's interferometers actually use multiple power recycling mirrors but for simplicity only one is shown in the diagram.

But there's a problem here too. LIGO's laser first enters the interferometer at about 200 Watts, but it operates closer to 750 kW when the instrument is at full-power. Again, just as it would be impossible to build a 1600 km long interferometer, building a laser with this initial power is a practical impossibility. Another paradox.

This problem is resolved with the help of "power recycling" mirrors. The figure at left shows schematically where a power recycling mirror is located.

Inside the interferometer, light from the laser passes through the transparent side of a "power recycling mirror" (think of the one-way mirror in your favorite cop show) to reach the beam splitter and then on to the arms of the interferometer. The instrument's alignment ensures that nearly all of the reflected laser light from the arms follows a path back to the laser, rather than straight to the photodetector. Laser light coming from the arms is reflected by the power recycling mirror back into the interferometer which greatly boosts the power of the laser beam without needing to generate such a powerful beam at the outset.

The boost in power generated by power recycling results in a sharpening of the interference fringes that appear when the two beams are superimposed--fringes which will tell scientists if a gravitational wave has passed. The sharper the fringes, the easier it becomes to identify the tell-tale signs of gravitational waves.

 

Two other modifications make LIGO's interferometers unique. First, they also possess 'signal recycling' mirrors, which like power recycling, enhances the signal that is received at the output of the instrument. And second, LIGO's interferometers were constructed with extraordinary mechanisms to damp out unwanted vibrations (noise) making it easier for scientists to weed out vibrations caused by gravitational waves. LIGO's seismic isolation system is discussed in much greater detail in LIGO Technology.

With these modifications, LIGO's interferometer is known as a Dual Recycled, Fabry-Perot Michelson -- but at its heart, it is still a Michelson interferometer.