Mid-infrared upconversion spectroscopy based on a Yb:fiber femtosecond laser

TL;DR

This paper introduces a broadband mid-infrared laser spectroscopy system using a Yb:fiber femtosecond laser for sensitive, high-resolution molecular detection with near-infrared upconversion, suitable for trace gas analysis.

Contribution

The work demonstrates a novel mid-infrared spectroscopy system with broad tunability, high resolution, and precise frequency calibration based on a Yb:fiber laser and upconversion detection.

Findings

Achieved MIR tunability from 2100-3700 cm^-1 with 40 mW power

Demonstrated detection limit of 2 parts-per-billion for gases

Achieved spectral resolution of 0.048 cm^-1 and frequency uncertainty below 0.005 cm^-1

Abstract

We present a system for molecular spectroscopy using a broadband mid-infrared laser with near infrared detection. Difference frequency generation of a Yb:fiber femtosecond laser produced a mid-infrared (MIR) source tunable from 2100-3700 cm^-1 (2.7-4.7 microns) with average power up to 40 mW. The MIR spectrum was upconverted to near-infrared wavelengths for broadband detection using a two-dimensional dispersion imaging technique. Absorption measurements were performed over bandwidths of 240 cm^-1 (7.2 THz) with 0.048 cm^-1 (1.4 GHz) resolution, and absolute frequency scale uncertainty was better than 0.005 cm^-1 (150 MHz). The minimum detectable absorption coefficient per spectral element was determined to be 4.4 x 10^-7 cm^-1 from measurements in low pressure CH_4, leading to a detection limit of 2 parts-per-billion. The spectral range, resolution, and frequency accuracy of this system show promise for determination of trace concentrations in gas mixtures containing both narrow and broad overlapping spectral features, and we demonstrate this in measurements of air and solvent samples.