  Squeezed light is injected into the dark port of gravitational wave
interferometers, in order to reduce the quantum noise. A fraction of the
interferometer output light can reach the OPO due to sub-optimal isolation of
the squeezing injection path. This backscattered light interacts with squeezed
light generation process, introducing additional measurement noise. We present
a theoretical description of the noise coupling mechanism. We propose a control
scheme to achieve a de-amplification of the backscattered light inside the OPO
with a consequent reduction of the noise caused by it. The scheme was
implemented at the GEO 600 detector and has proven to be crucial in maintaining
a good level of quantum noise reduction of the interferometer for high
parametric gain of the OPO. In particular, the mitigation of the backscattered
light noise helped in reaching 6dB of quantum noise reduction [Phys. Rev. Lett.
126, 041102 (2021)]. The impact of backscattered-light-induced noise on the
squeezing performance is phenomenologically equivalent to increased phase noise
of the squeezing angle control. The results discussed in this paper provide a
way for a more accurate estimation of the residual phase noise of the squeezed
light field.
