A highly stable 2 W Nd:YAG Laser as the "Work Horse" for Gravitational-wave Detection
These GWDs need to measure a relative differential length change of perpendicular interferometer arms with a precision of ∆l/l≈10-22. This corresponds to measuring the distance between Earth and Sun to the diameter of a single atom. It is not surprising that an extremely stable laser source is required to illuminate these interferometers.
From gas lasers to solid-state NPROs
Early GWD prototypes in the 1980s used argon-ion (Ar+) lasers as their light source. The invention of the non-planar ring-oscillator (NPRO) by Byer and Kane  in 1985 initiated a change from gas lasers to solid-state lasers as GWD light sources. NPROs offered much better wall-plug efficiency and a noise level several orders of magnitude lower than Ar+ lasers. For these reasons and as NPROs were commercially available from two companies (Lightwave USA, Innolight Germany), all proposed and later build kilometer-scale GWDs used Nd:YAG solid state lasers based on NPROs as their main light source.
Today NPROs are not only used as master/seed lasers in the high-power main light source of GWDs but as well to generate non-classical states of light (so-called squeezed states) to overcome the shot-noise limit in the GWD readout. They are also used as seed lasers for frequency-doubled laser systems for the arm-length stabilization of GWDs and in many other fields of high precision metrology and quantum communication.