  A dual-pass differential Fabry-Perot interferometer (DPDFPI) is one candidate
of the interferometer configurations utilized in future Fabry-Perot type space
gravitational wave antennas, such as Deci-hertz Interferometer Gravitational
Wave Observatory. In this paper, the working principle of the DPDFPI has been
investigated and necessity to adjust the absolute length of the cavity for the
operation of the DPDFPI has been found. In addition, using the 55-cm-long
prototype, the operation of the DPDFPI has been demonstrated for the first time
and it has been confirmed that the adjustment of the absolute arm length
reduces the cavity detuning as expected. This work provides the proof of
concept of the DPDFPI for application to the future Fabry-Perot type space
gravitational wave antennas.
