  Gravitational waves were first proposed by Henri Poincar\'e in 1905 and were
subsequently predicted by Albert Einstein in his General Theory of Relativity.
In 2015, first Gravitational Waves signals were detected by LIGO of two black
holes merging about 1.3 billion light-years away. The discovery opened a new
window of astronomy. Currently, four laser interferometers are operating around
the world, two American detectors which are located in Hanford and Livingston,
one Italian detector (VIRGO) located at Pisa and an underground Japanese
detector KAGRA. This paper describes the modern optical techniques that are
being used and future planned techniques for gravitational wave detection.
