LIGO's Interferometer

Basic Michelson Labeled

A basic Michelson interferometer

 

Although 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 traits:

  • 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 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 the instrument becomes. Attempting to measure a change in arm length 1,000 times smaller than a proton means that LIGO has to be more sensitive than any scientific instrument ever built, so the longer the better. But there are obvious limitations to how long one can build an interferometer. Even with arms 4 km long, if LIGO's interferometers were basic Michelsons 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 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 dilemma was fixed by adding something called "Fabry Perot cavities" to the basic Michelson design. The figure at left shows how a basic Michelson interferometer is modified to include Fabry Perot cavities. Compare this to the figure above.Each arm has a Fabry Perot 'cavity'. It is created by adding mirrors near the beam splitter that continually reflect parts of each laser beam back and forth within the 4 km long arms about 280 times before they are merged together again.

With Fabry Perot cavities, LIGO's interfereometer arms are effectively 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, all while keeping the physical size of the interferometer manageable.

Those familiar with 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 photons that merge at the beam splitter, the sharper the resulting interference pattern becomes, making it 'easier' to recognize a gravitational wave signature.

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 itself, 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 first passes through the mirror to reach the beam splitter where it is split and directed down the arms of the interferometer. The instrument is aligned so well 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 ends of the arms is thereby reflected back into the interferometer (hence 'recycling') where those photons add to the ones just 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 this 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 and vastly different from a Michelson interferometer. First, they also possess 'signal recycling' mirrors, which like power recycling, enhance the output signal. And second, LIGO's interferometers can damp out unwanted vibrations (noise) making it easier for scientists to recognize the tell-tale vibrations caused by gravitational waves. Since this vibration damping is such a critical part of LIGO's operation, this topic is discussed in much greater detail in LIGO Technology.

Putting all of these upgrades together, LIGO's interferometer is technically known as a Dual Recycled, Fabry-Perot Michelson Interferometer, or DRFPMI.