Free-Running Ring Quantum Cascade Laser with 50 kHz Linewidth

TL;DR

This paper characterizes a free-running ring quantum cascade laser emitting around 7.7 μm, achieving a linewidth of approximately 50 kHz, significantly improving spectral purity and enabling advanced mid-infrared spectroscopy.

Contribution

It reports the first detailed noise characterization and linewidth measurement of a free-running ring quantum cascade laser in this spectral region.

Findings

Linewidth close to 50 kHz at 1 s integration time

Sixfold improvement over previous quantum cascade lasers above 7 μm

Demonstrated suitability for high-resolution mid-infrared spectroscopy

Abstract

We report on the noise characterization of a free-running ring quantum cascade laser resonator emitting a single frequency mode around 7.7 $\mu$m. Using a gas cell filled with N$_2$O as a frequency-to-voltage discriminator, we measured the frequency noise power spectral density of the laser from which we extracted its linewidth. The results show a full width at half maximum close to 50 kHz at 1 s integration time, which represents at least a sixfold improvement compared to state-of-the-art quantum cascade lasers operating in a spectral region above 7 $\mu$m. We also demonstrate that such lasers can be efficiently used for frequency modulation spectroscopy, which opens up new possibilities for high resolution metrology and spectroscopic applications in the mid-infrared.