To respond to the demand for the calibration of precision linear encoders, we have developed a new length calibrator with sub-nanometer uncertainty. It is capable of calibrating the non-linearity errors of the precision linear encoders with the best measurement uncertainty of 0.6 nm (k=2). The system consists of an optical zooming interferometer, which can realize a several picometer resolution and sub-nanometer accuracy of positioning. Two tunable diode lasers, the optical sources of this interferometer, are stabilized by using an optical frequency comb. The calibration services for the precision linear encoders have been started.