Lasers play an important role in measuring distances.The national standard of length in Japan changed to a femtosecond optical frequency comb(FOFC)in July 2009.The center frequency of the FOFC standard is 1560 nm,which is in the transmission band of the optical fiber.Thanks to this fact,an FOFC can be delivered to various locations,such as factories and laboratories,through the optical fiber networks.That means that everyone at the measurement site can access the high-accuracy length standard via fiber networks.The standard transmission medium for telephone calls(known as telecom)and that for data(known as datacom)is currently optical fibers.Thus,we can expect optical fibers will also be the new standard intermediary for the meter,which we therefore refer to as metercom.Length measurements on production sites are often performed in an open environment.It is difficult to accurately measure the temperature,the atmospheric pressure,and the humidity along a particular optical path.This is particularly true for a long-path interferometer or an interferometer in an uncontrolled/uncontrollable environment.As an alternative to the conventional two-color(namely,wavelength)absolute length-measurement technique,we propose the adjacent pulse repetition interval length(APRIL)-based two-color method.This two-color method uses an APRIL(the physical length associated with the pulse repetition period)as a ruler for measuring length.A demonstration(based on numerical simulations)of the proposed method is presented.From the result of the demonstration,it has been show the feasibility of the proposed method.