  The quantum noise in gravitational-wave detectors can be suppressed in a
broadband by frequency-dependent squeezing. It usually requires one large scale
filter cavity and even two, for example in the low frequency detector of
Einstein Telescope, which is a detuned dual recycling Fabry-Perot Michelson
interferometer. In this paper, we study the feasibility of replacing two filter
cavities with a coupled-cavity, aiming to reduce the optical losses with less
number of optics. It turns out this approach is only theoretically valid,
however, the required parameters of the optics don't support practical
implementation, which is consistent with the results in [Phys. Rev. D {\bf
101}, 082002 (2020)]. Furthermore, we investigate the viability of utilizing
EPR squeezing to eliminate either one or two filter cavities in Einstein
Telescope. It turns out EPR squeezing would allow to eliminate one filter
cavity, and can potentially improve the detector sensitivity with the allowance
of higher input squeezing level, benefiting from the longer length of the arm
cavity which serves as one of the filter cavities for frequency-dependent
squeezing.
