30 Hz-linewidth, diode laser-pumped, Nd:GGG nonplanar ring oscillators by active frequency stabilisation

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Abstract
We report a heterodyne linewidth of less than 30 Hz for the beatnote between the outputs of two 282 THz (1.062µm) Nd:GGG nonplanar ring oscillators (NPROs). The lasers were independently locked to adjacent axial modes of a highfinesse interferometer. The remnant frequency noise appears to be dominated by free spectral range fluctuations in the reference interferometer rather than by residual laser noise.