Fig. 1. Pump selection of the thulium-doped fiber. (a) Simplified energy level bands of the Tm³⁺. (b) Absorption and emission cross-section spectra of thulium^[31]. GSA, ground state absorption; ESA, excited state absorption.

Fig. 2. Schematic of the experimental setup.

Fig. 3. Absorption line selection for CO₂ detection. (a) CO₂ and H₂O molecular absorption lines at the wavelength band of 2000–2005 nm; (b) second-harmonic signals of different pump powers at 2004 nm; (c) second-harmonic amplitudes of different CO₂ absorption lines in different pump powers.

Fig. 4. Second-harmonic signals in different concentrations based on 3 W pump power. Inset: second-harmonic signal of 500 ppm CO₂ without a pump.

Fig. 5. Performance analysis. (a) Linearity of CO₂ detection in concentration range of 0–10,000 ppm. (b) Linearity of CO₂ detection in low concentration range of 0–500 ppm. (c) Non-absorption noise measured in pure N₂.