Introduction
To begin with, we need to learn something about some basic knowledge of LIGO and gravitational waves. LIGO is the abbreviate for Laser Interferometer Gravitational-Wave Observatory with the aim of detecting gravitational waves from the universe. The gravitational wave is a concept predicted by Albert Einstein in his general theory of relativity. Generally speaking, it is the disturbance of spacetime, just like the ripples caused by a drop of water. However, the "ripples" are too small in amplitude which brings huge difficulty in detection. LIGO was explicitly designed with interferomerty tech to solve the problem. We will discuss it in detail in the following paragraph.
A. History of LIGO
Its history could be traced back to the 1970s, when many physicists still believed in using Weber bar--a large solid bar designed by Joseph Weber based on the principle of resonance including Kip Thorne, one of the leading starters of LIGO.
Kip Thorne spent a lot of time persuading the physics and astronomy department of Caltech to build a team to design a gravitational detector. Thanks to Ronald Drever from University of Glasgow who convinced Thorne of the limitation of Weber bar and transfer his the idea to the detector using the principle of Michelson interferometer.
In 1986, under the suggestion of National Science Foundation(NSF), the Caltech group led by Thorne and Drever and the MIT group led by Rainer Weiss began the huge scientific project LIGO. \cite{caltech2016}\cite{Cho_2016}
B. Basic Principle
The primary interferometer consists of two beam lines of 4 km length which form a power-recycled Michelson interferometer with Gires–Tournois etalon arms.
Interferometer:
It is the core technology of LIGO, which is based on the principle of Michelson interferometer.