Extremely high-accuracy correction of air refractive index using two-colour optical frequency combs

Abstract

Optical frequency combs have become an essential tool for distance metrology, showing great advantages compared with traditional laser interferometry. However, there is not yet an appropriate method for air refractive index correction to ensure the high performance of such techniques when they are applied in air. In this study, we developed a novel heterodyne interferometry technique based on two-colour frequency combs for air refractive index correction. In continuous 500-second tests, a stability of 1.0 × 10−11 was achieved in the measurement of the difference in the optical distance between two wavelengths. Furthermore, the measurement results and the calculations are in nearly perfect agreement, with a standard deviation of 3.8 × 10−11 throughout the 10-hour period. The final two-colour correction of the refractive index of air over a path length of 61 m was demonstrated to exhibit an uncertainty better than 1.4 × 10−8, which is the best result ever reported without precise knowledge of environmental parameters.

Correction of group refraction index based on pulse trains interference

“…For convenience, we summarize the basic concepts of the wavelength-based two-color method; the details of this method can be obtained elsewhere (1)(2)(3)(4).…”

Correction of refraction index based on adjacent pulse repetition interval lengths

“…Since the early stage of the two-color method [3], many contributions have been made to correction of the atmospheric index of refraction [4][5][6][7][8][9][10][11][12][13][14][15][16][17].…”

Absolute distance measurement with extension of nonambiguity range using the frequency comb of a femtosecond laser

“…[19][20][21] Finally, Fig. When the measurement is made in air, jδDj is also affected by the refractive index of air with Λ CV being subsequently shortened further.…”