Fig. 1. Schematic of 4F degenerate cavity

Fig. 2. Experimental configuration

Fig. 3. Influence of cavity length variation on focal length of thermal lens

Fig. 4. Influence of degenerate cavity length on time distribution characteristic.(a)Beat frequency distribution of degenerate cavity;(b)influence of cavity length variation on beat bandwidth of transverse mode

Fig. 5. Influence of cavity length variation on maximum order of transverse mode

Fig. 6. Influence of cavity length variation on frequency interval of transverse mode and fundamental moderadius under thermal lens