  Direct observations of gravitational waves at frequencies below 10 Hz will
play crucial roles for fully exploiting the potential of gravitational wave
astronomy. One approach to pursue this direction is the utilization of laser
interferometers equipped with the Fabry-Perot optical cavities in space.
However, a number of challenges lie in this path practically. In particular,
the implementation of precision control for the cavity lengths and the
suppression of laser phase noises may prevent a practical detector design. To
circumvent such difficulties, we propose a new interferometer topology, named
the back-linked Fabry-Perot interferometer, where the precision length controls
are not required and an offline subtraction scheme for laser phase noises is
readily applicable. This article presents the principle idea and the associated
sensitivity analyses. Despite additional noises, a strain sensitivity of
$7\times 10^{-23}$ Hz$^{-1/2}$ may be attainable in the deci-Hertz band.
Several technological developments and studies must be carried out to pave the
way forward for the implementation.
