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.