  We present an absolute distance measurement interferometer based on a two
wavelength interferometer and a variable synthetic wavelength technique. The
wavelength scanning range was 12 GHz, realized with a phase accuracy of 1.0
m{\lambda} by heterodyne detection at each measurement wavelength. This small
wavelength scanning range enabled the use of distributed feedback laser diodes
as an interferometer light source and a fast 20 ms wavelength scanning time by
injection current control. We demonstrated a measurement range of up to 1.5 m
and an accuracy better than 1.2 nm in comparison with a displacement
measurement interferometer, corresponding to a relative accuracy of 10-9. In
addition, we also proposed expanding the range of maximum measurement and
compensation of refractive index of air for linear colliders.
