Fig. 1. (a) Acoustic pressure distribution of the traditional off-beam QEPAS system. (b) Acoustic pressure distribution of the single off-beam exciting dual-QTF system. (c) QTF displacement of the traditional off-beam QEPAS system. (d) QTF displacement of the single off-beam exciting dual-QTF system.

Fig. 2. Diagram of experimental setup. DFB laser, distributed feedback laser; FC, fiber collimator; AmR, acoustic micro-resonator; QTF, quartz tuning fork; PC, personal computer. (a) Top view of the single off-beam AmR and dual-QTF configuration. (b) Top view of the traditional off-beam configuration.

Fig. 3. Frequency response curves of QTF1 and QTF2.

Fig. 4. 2f signal amplitude of QTF1 versus modulation current.

Fig. 5. Relationship between the 2f signal amplitude and the length of AmR for the off-beam QEPAS sensor with a single-hole AmR.

Fig. 6. Comparison of 2f signals among off-beam QEPAS systems with single-hole AmR and dual-hole AmR, and the bare QTF system.

Fig. 7. Noise and SNR for five combinations. (a) Dual-hole based QTF1 + QTF2. (b) Single-hole based QTF1. (c) Dual-hole based QTF1. (d) Dual-hole based QTF2. (e) Bare QTF.