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

Area of Science:

• Optics and Photonics
• Metrology
• Interferometry

Background:

• Optical frequency combs offer significant advantages over traditional laser interferometry for distance metrology.
• A critical challenge remains the lack of effective air refractive index correction methods for high-performance optical metrology in ambient air.

Purpose of the Study:

• To develop a novel technique for accurate air refractive index correction in optical metrology.
• To enhance the performance of distance metrology techniques when operating in air using optical frequency combs.

Main Methods:

• Development of a novel heterodyne interferometry technique.
• Utilization of two-colour optical frequency combs for simultaneous measurement.
• Application of the technique for air refractive index correction over a 61 m path length.

Main Results:

• Achieved a measurement stability of 1.0 × 10⁻¹¹ over 500 seconds for the optical distance difference between two wavelengths.
• Demonstrated agreement between measurement results and calculations with a standard deviation of 3.8 × 10⁻¹¹ over 10 hours.
• Obtained an uncertainty better than 1.4 × 10⁻⁸ for two-colour refractive index correction of air, without requiring precise environmental data.

Conclusions:

• The developed two-colour frequency comb heterodyne interferometry technique provides a robust solution for air refractive index correction.
• This method significantly improves the accuracy and reliability of optical distance metrology in ambient air.
• It represents the best reported uncertainty for refractive index correction without precise environmental parameter knowledge.