Fig. 1. Straight cavity structure optical cavity ring-down system

Fig. 2. Four high-order transverse modes generated by cavity imbalance. (a) TEM11; (b) TEM21; (c) TEM12; (d) TEM03

Fig. 3. Spot modes when adjusting cavity mirror. (a) Adjust cavity mirror in the vertical direction only; (b) adjust avity mirror in the horizontal direction only

Fig. 4. Area ratio of different spot modes

Fig. 5. Flow chart of cavity adjustment process

Fig. 6. Schematic of the initial cavity experimental device

Fig. 7. Schematic of the test cavity experimental device

Fig. 8. Initial cavity performance parameters in different modes

Fig. 9. Performance parameters of test cavity in different modes

Fig. 10. Reflectance of sample corresponding to several special spot modes in Table 2