In this paper, we present a laser-based sensing inside anti-resonant hollow core fiber. A distributed feedback laser diode operating near 2004 nm and a 1.35-m-long silica-based fiber are used to demonstrate carbon dioxide detection with sensitivity down to ~5 ppmv. Gas exchange time as low as 5 seconds is obtained. This performance was achieved in a very simple optical configuration, without any mirrors or lenses in the setup.