We present a phase-locked synthetic wavelength interferometer that enables a complete elimination of cyclic errors in absolute distance measurements. With this method, the phase difference between the reference and measurement paths is fed back into a phase lock-in system, which is then used to control the synthetic wavelength and set the phase difference to zero using an external cavity acousto-optic modulator. We validated the cyclic error removal of the proposed phase-locked method by comparing it with the conventional phase-measuring method of the synthetic wavelength interferometer. By analyzing the locked error signal, we achieved a precision of 0.6 mrad in phase without any observed cyclic errors.

A synthetic wavelength interferometer (SWI) with a long period wave generated through interference of waves which have similar frequencies has an advantage of avoiding phase ambiguity in long distance measurement. However, during measuring the distance, a cyclic error occurs that deteriorate the resolution and accuracy. In this study, the synthetic waves having frequencies from tens of MHz to a GHz are generated by employing an Acousto-Optic Modulator (AOM) and a femtosecond laser. In addition, a phase locked method is proposed to remove cyclic error. The phase locked method is locking a difference of phase between measurement arm and reference arm of the SWI to a constant value. This can be implemented by utilizing a P-I controller to maintain voltage resulting from the phase difference between arms at a certain value. An output voltage from the P-I controller was converted to oscillation frequency using Voltage-Controlled Oscillator (VCO) principle. This oscillation frequency was input in the AOM as a modulation frequency. As a result, it is possible to lock the phase to a constant value by modulating the frequency of synthetic wave. It can also be used to measure the absolute distance by selecting a specific frequency of the synthetic wave. The advantage of this method is that the SWI can have high resolution over a wide range of distance measurement depending on the frequency, as well as eliminate cyclic error.