Large-scale and high precision absolute distance measurement is essential in aerospace technology and advanced manufacturing. Traditional method of measuring distance cannot meet this requirement. Since the advent of optical frequency comb, it has brought a revolutionary breakthrough to absolute distance measurement. In the past decade, there were proposed many methods to measure long absolute distances with high accuracy. Especially, the simple method of using adjacent pulse-to-pulse distance as a ruler for distance measurement has been widely used. The accuracy of this method depends mainly on the knowledge of relative positions of the two overlapped pulses, i.e., pulse-to-pulse alignment. In our previous study, we have proposed a heterodyne interferometer based on synthetic wavelength method with femtosecond laser. The synthetic wavelength is derived from the virtual second harmonic and the real second harmonic, and the real second harmonic is produced by a piece of periodically poled LiNbO3 (PPLN) crystal. However, the second harmonic generation system makes the system complicated, and causes a great optical energy loss. In order to solve this problem, we generate the synthetic wavelength by two spatial band-pass filters in our present study, which can simplify the system greatly. Moreover, we can reduce the optical energy loss and tune the synthetic wavelength by controlling the angle of the filter. The synthetic wavelength used in the present system is 71.39 m. The interferometric phase of the synthetic wavelength is used as a mark for the pulse-to-pulse alignment. In order to reduce the influences of air disturbance and temperature variation, we set up a thermal-insulated cover for the interferometer to stabilize the environment in the system. By using this cover, the optical path length difference of the system in 450 s can be reduced from 8.56 m to 0.21 m. To demonstrate the efficacy of the method described above, the target mirror is moved by eight steps in steps of 5 mm. We compare the measurement results with those obtained by a commercial interferometer, and the residual error is less than 100 nm. Since the measurement range is larger than our previous study, the relative accuracy is better than the previous system. In conclusion, we demonstrate a synthetic-wavelength based absolute distance measurement by using heterodyne interferometry of a femtosecond laser. Two spatial band-pass filters are used to generate the synthetic wavelength, which can simplify the system. The comparison results show that the system has an accuracy better than 100 nm in a displacement of 40 mm. The accuracy of the experimental system can be further improved by making the common-path of the two interferometers longer, locking the fceo to the atomic clock and sampling the data synchronously.
