LIGO's Interferometer

Basic Michelson Labeled

Configuration of a basic Michelson interferometer

Although they are much more sophisticated, at their cores, LIGO's interferometers are fundamentally Michelson Interferometers, a device invented in the 1880's. We can say this because both Michelson and LIGO interferometers share these characteristics:

  • They are both are L-shaped (not all interferometers are this shape)
  • They both have mirrors at the ends of the arms to reflect light in order to combine light beams and create an interference pattern
  • They both measure patterns and intensity of a resulting light beam after two beams have been superimposed or forced to 'interfere'

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.

 

World's Largest and Most Sensitive

LIGO's interferometers are by far the largest ever built. With arms 4 km (2.5 mi.) long, they are 360 times larger than the one used in the Michelson-Morley experiment (which had arms 11 m (33 feet) long).

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 to length changes the instrument becomes. Attempting to measure a change in arm length 10,000 times smaller than a proton meant that LIGO had to be more sensitive than any scientific instrument ever built, so the longer the better. But there are obvious practical limitations to how long one can build an interferometer. Even with arms 4 km long, if LIGO's interferometers were basic Michelsons (with one reflection of light), they would still not be long enough to detect gravitational waves...and yet they are. How is this possible?

Basic Michelson with FP labeled

Basic Michelson interferometer with Fabry Perot cavities. Mirrors placed near the beam splitter cause multiple reflections and keep the laser contained within the arms. This increases the distance traveled by the beams, greatly improving LIGO's sensitivity to changes in arm length like those caused by gravitational waves.

This obstacle was resolved by altering the design of the Michelson to include modifications called "Fabry Perot cavities." The figure at left shows a basic Michelson interferometer that has been modified to include Fabry Perot cavities. Compare this to the figure above.

The Fabry Perot 'cavity' actually is the full 4 km length of each arm between the beam splitter and the end of each arm. Additional mirrors placed near the beam splitter are precisely aligned to reflect each laser beam back and forth along this 4 km length about 280 times before it finally merges with the beam from the other arm. These extra 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 increases the distance traveled by each laser beam from 4 km to 1120 km

With Fabry Perot cavities, LIGO's interfereometer arms are not just 4 km long, they are essentially 1120 km long, making them 144,000 times bigger than Michelson's original instrument! This bit of 'mirror magic' greatly increases LIGO's sensitivity and makes it capable of detecting changes in arm-length thousands of times smaller than a proton, while keeping the physical size of the interferometer manageable.

Those familiar with optical telescopes will recognize this effect. Increasing a telescope's focal length doesn't just increase the magnification of any given eyepiece, it also magnifies the tiniest vibrations making them visible in the eyepiece; the longer the focal length, the smaller the vibration you see in the eyepiece. In a telescope, these vibrations are unwelcome, but LIGO is designed to feel them. And at effectively 1120 km long, LIGO's arms can readily magnify the smallest conceivable vibrations enough that they are measurable.

 

Power Boosted Laser

Length isn't the only design factor important to LIGO's sensitivity; laser power is too. While increasing length increases the interferometer's sensitivity to vibrations, increasing laser power improves the interferometer's resolution. The more laser photons there are moving through each arm and merging at the beam splitter, the sharper the resulting interference pattern becomes in the photodector, which in turn makes it 'easier' to recognize the signature flicker of gravitational waves.

Basic Michelson with FP and PR labeled

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

But there's a problem here too. For LIGO to operate at full sensitivity, its laser has to shine at 750 kilowatts. But LIGO's laser enters the interferometer at 200 Watts. And just as it is impossible to build a 1120 km-long interferometer, building a 750 kW laser is also a practical impossibility. So how does LIGO boost the power of its laser 3750 times without actually using more power?

More mirrors! Specifically, "power recycling" mirrors placed between the laser source and the beam splitter. Like the beam splitter, the power recycling mirror is only partly reflective (a 'one-way mirror'). The figure at left shows schematically where such a mirror is located.

In a power recycling mirror, light from the laser passes through the transparent side of the mirror to reach the beam splitter where it is split and directed down 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 recycling mirrors rather than to the photodetector. Laser light coming from the arms is reflected back into the interferometer (hence 'recycling') where those photons add to the ones first entering. This process greatly boosts the power of the beam without needing to generate a 750 kW beam at the outset.

The boost in power generated by power recycling sharpens the interference pattern that appears when the two beams are superimposed--the pattern that will tell scientists if a gravitational wave has passed. The sharper the pattern, the easier it becomes to recognize the fingerprints of gravitational waves.

Two other modifications make LIGO's interferometers unique. First, they also possess 'signal recycling' mirrors, which like power recycling, enhance the output signal. And second, but most important, LIGO's interferometers can damp out unwanted vibrations (noise) making it easier for scientists to weed out vibrations caused by gravitational waves. Since this is such a critical part of LIGO's operation, its seismic isolation system is discussed in much greater detail in LIGO Technology.

With these modifications, LIGO's interferometer is classified as a Dual Recycled, Fabry-Perot Michelson Interferometer.