Arbitrary distance measurement without dead zone by chirped pulse spectrally interferometry using a femtosecond optical frequency comb.

We demonstrate an arbitrary distance measurement method by chirped pulse spectrally interferometry (CPSI) using femtosecond optical frequency comb (OFC). In this paper, the chirped fiber Bragg grating (CFBG) is used to investigate the mapping relationship between displacement and the center frequency of the chirped spectral interferogram. We overcome the direction ambiguity of dispersive interferometry (DPI) ranging and expand the range of distance measurement to 18 cm. Besides, we achieve a full range of dead-zone free ranging by introducing a variable optical delay line (VODL). And through principles simulation and experiment, it is demonstrated that the measurement accuracy is 12 µm in comparison with an incremental He-Ne laser interferometer and the minimum Allen deviation is 52 nm at an average time of 1.76 ms. Similarly, in the experiment with long-distance of ∼30m, the accuracy reaches 20 µm, and 2.51 µm repeatability is achieved under harsh environment.

Improvement of Distance Measurement Based on Dispersive Interferometry Using Femtosecond Optical Frequency Comb.

Since the dispersive interferometry (DPI) based on optical frequency combs (OFCs) was proposed, it has been widely used in absolute distance measurements with long-distance and high precision. However, it has a serious problem for the traditional DPI based on the mode-locked OFC. The error of measurements caused by using the fast Fourier transform (FFT) algorithm to process signals cannot be overcome, which is due to the non-uniform sampling intervals in the frequency domain of spectrometers. Therefore, in this paper, we propose a new mathematical model with a simple form of OFC to simulate and analyze various properties of the OFC and the principle of DPI. Moreover, we carry out an experimental verification, in which we adopt the Lomb-Scargle algorithm to improve the accuracy of measurements of DPI. The results show that the Lomb-Scargle algorithm can effectively reduce the error caused by the resolution, and the error of absolute distance measurement is less than 12 µm in the distance of 70 m based on the mode-locked OFC.

Nonequal arm surface measurement of femtosecond optical frequency combs using the Savitzky-Golay filtering algorithm.

In precision machining, the surface geometry of a device is one of the important parameters that directly affects the device performance. This paper proposes nonequal arm surface measurement of femtosecond optical frequency combs (OFCs) using the Savitzky-Golay filtering algorithm, which uses the high spatial coherence of OFCs to realize high-precision, nonequal surface measurements. The Savitzky-Golay filtering algorithm and a high-order polynomial envelope fitting algorithm are used to smooth and denoise the interference signals to improve signal quality and measurement accuracy. The experiments are carried out under the condition of nonequal arms, and the results show that the repeatability is 28.6 nm for 20 consecutive measurements on the step surface of a 0.5 mm gauge block. The frosted glass surface is measured 20 times, and the measurement repeatability at the center position is 89.6 nm, which verified the system capability of nonequal arm high-precision measurement under different reflective surfaces.