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.
Low frequency and power noise with NPROs
NPROs are laser diode pumped, monolithic ring-oscillators that are available with output powers of up to 2 W (Mephisto family, Coherent). The non-planar beam path in combination with an internal Faraday rotation of the polarization direction and polarization dependent losses cause a linear polarized, single directional operation of the ring laser. The monolithic design reduces vibrations of the resonator mirrors and hence leads to very high frequency stability. Furthermore, a low-noise design of the pump light electronics reduces the power noise of the laser. A specific resonator design leads to a single frequency operation in the fundamental Gaussian mode.
In addition to their low-noise performance the commercial versions of NPROs have fast, large-range actuators for frequency and power control. They also feature an optional internal feed-back control system to reduce the power noise at the relaxation oscillation frequency which – depending on the model – lies between 100 kHz and 1 MHz. A detailed comparative noise characterization of eight commercial NPRO lasers by Kwee et. al.  showed a high unit-by-unit reproducibility and a stable operation of one units for 3.5 months.
Power amplification for gravitational-wave detectors
As GWDs need more light power than NPROs can deliver, two concepts were used in the first generation of GWDs: a master-oscillator power-amplifier design for LIGO  and injection-locked master-slave systems for GEO600 , Virgo  and TAMA300 . Both concepts increase the power and preserve the frequency stability of the NPRO master laser. Even though the NPRO's frequency noise is among the lowest of commercially available Nd:YAG lasers, GWDs require a reduction of this noise by several orders of magnitude. This is achieved by nested feed-back control loops which use the NPRO's frequency control actuators (a fast actuator up to 100 kHz and a long-range slow actuator). An overview of the laser stabilization for GWDs can be found in reference .
The first generation of GWDs was operated continuously for 24 hours a day, seven days a week for more than 10 years. During this time the different NPRO master lasers (different vendor and different output power levels) worked stably and reliably.
NPROs as highly reliable laser sources
Since 2000 a team of scientists and engineers at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute), at the Laser Zentrum Hannover, and at LIGO have been working on the design, fabrication, and implementation of a 200 W laser system for the second generation Advanced LIGO GWD [8,9]. As NPROs have proven to be reliable, low-noise lasers in the first generation of GWDs, the team again relies on a NPRO master laser (Mephisto2000, Coherent). Even though the Advanced LIGO GWDs will start operation not before 2015, four pre-stabilized 200 W laser systems have been in continuous operation since 2010. Again the NPROs have proven very reliable as documented by the sophisticated digital monitoring system of Advanced LIGO.
At the GEO600 detector NPROs have been in use since 1998: between 1998 and 2011 in a master-slave configuration  and since October 2011 as a seed laser for a 35 W amplifier system . Since the summer of 2010 NPROs serve an additional purpose in the GEO600 GWD: Three of them are used in compact experimental unit that delivers a non-classical state of light (squeezed vacuum) to overcome the shot-noise limit at the GW readout port of GEO600. (More details about the squeezer and its implementation in GEO600 can be found in references  and .) This is another laser application in GWDs that requires low-noise laser sources which the NPROs have been reliably providing for years.
Due to their low-noise performance and high reliability NPRO lasers were successfully operated for many years in all GWDs of the first generation and in many research labs of the GW community. They are the preferred seed-laser source for GWDs of the second generation which is currently been commissioned and they might also be used in third-generation GWDs. They are an excellent choice for precision experiments that require continuous-wave single-mode single-frequency laser sources at 1064 nm.