The idea of using laser interferometer for detecting gravitational waves was first proposed in 1962 (shortly after inventing lasers) by M. Gertsenshtein and V. Pustovoit [1]. So it took more than 50 years from the idea to its implementation.

[1] http://www.jetp.ac.ru/cgi-bin/dn/r_043_0605.pdf (in Russian, Abstract in English at the end).