# Raman Gas Analysis with External Power Build-Up Cavity of Line-Narrowed 407-nm Laser Diode

## TL;DR

A line-narrowed 407-nm laser diode with an external cavity was used for sensitive Raman gas detection, achieving a methane detection limit of 1 ppm.

## Contribution

A novel external power build-up cavity setup using a line-narrowed laser diode improves Raman gas detection capabilities.

## Key findings

- The laser linewidth was effectively narrowed using resonant optical feedback in an external cavity.
- Tens of watts of power were achieved in the external cavity, enhancing detection performance.
- A methane detection limit of 1 ppm was achieved in ambient air analysis.

## Abstract

An external power build-up cavity of a line-narrowed 407-nm laser diode for Raman gas analysis was demonstrated to possess good gas detection capabilities. By employing an ordinary laser diode without anti-reflection coating or and a bandpass interference filter in an external cavity resonance, the laser linewidth was narrowed by resonant optical feedback, and tens of watts of external cavity power were built up. The coupling mechanism between the semiconductor laser and the external cavity are discussed, as well as the noise background in the experimental results. The Raman spectrum of ambient air was analyzed, achieving a methane detection limit of 1 ppm.