High-precision diode-laser-based temperature measurement for air refractive index compensation

Area of Science:

• Optics and Photonics
• Atmospheric Physics
• Spectroscopy

Background:

• Accurate refractive index of air is critical for optical distance measurements.
• Traditional methods rely on ambient air properties, with temperature being a major uncertainty source.
• Existing methods often lack spatially matched environmental data.

Purpose of the Study:

• To develop a laser-based system for measuring the refractive index of air over long path lengths.
• To improve the accuracy of environmental measurements (temperature, humidity) for optical metrology.
• To demonstrate effective compensation of air refractive index variations in interferometric measurements.

Main Methods:

• Utilized direct absorption spectroscopy of oxygen to measure average air temperature.
• Employed spectroscopy of water vapor to determine relative humidity.
• Integrated environmental sensing along the same optical path as distance measurement.

Main Results:

• Demonstrated effective compensation of air refractive index in interferometric length measurements under challenging conditions.
• Achieved 7 mK RMS noise in temperature measurement over a 67 m path length with a 120 s sample time.
• Reported the best temperature precision for spectroscopic temperature measurement to date.

Conclusions:

• The developed spectroscopic method provides accurate, spatially matched temperature and humidity data for optical distance measurements.
• This technique significantly enhances the precision of long-path optical measurements by compensating for environmental variations.
• The system offers a breakthrough in spectroscopic temperature sensing accuracy.