Currently, the German/British gravitational wave (GW) detector GEO 600 is being upgraded within the GEO-HF program. One part of this upgrade consists of the integration of a squeezed-light laser to non-classically improve the detection sensitivity at frequencies where the instrument is limited by shot noise. This has been achieved recently (Abadie et al 2011 Nature Phys. 7 962). The permanent employment of squeezed light in GW observatories requires long-term stability of the generated squeezed state. In this paper, we discuss an unwanted mechanism that can lead to a varying squeezing factor along with a changing phase of the squeezed field. We present an extension of the implemented coherent control scheme (Vahlbruch et al 2006 Phys. Rev. Lett. 97 011101) that allowed an increase in the long-term stability of the GEO 600 squeezed-light laser. With it, a quantum noise reduction of more than 9 dB within the detection band of today's and next-generation GW observatories was observed up to 20 h with a duty cycle of more than 99%.
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