Fig. 1. Schematics of IR-FEL. (a) Structural diagram of POP-IN detector; (b) upstream mirror; (c) downstream mirror

Fig. 2. Influence of lateral tilt of cavity mirror on output FEL intensity under different working wavelengths of resonant cavity. (a) 2.5 μm; (b) 50 μm

Fig. 3. Alignment adjustments of upstream and downstream mirrors of resonant cavity based on laser autocollimator. (a) Light path diagram of laser autocollimator; (b) measurement result of downstream mirror tilt; (c)measurement result of upstream mirror tilt

Fig. 4. Schematic of optical path of laser online collimation system

Fig. 5. Measuring 1 mm hole (C) on POP-IN by target ball (A and B) and collimating telescope

Fig. 6. Physical details of on-line collimation system. (a) Lase and reflector; (b) membrance reflector; (c) flat mirror; (d) membrance reflectors for far and mid-infrared lasers