Fig. 1. Longitudinal mode distribution of He-Ne laser tube

Fig. 2. Gain-frequency curve of He-Ne laser tube (double longitudinal mode)

Fig. 3. Structural diagram of frequency stabilization for double longitudinal mode thermal frequency stabilization method

Fig. 4. Voltage error between two longitudinal modes and laser tube temperature versus time

Fig. 5. Relationship between wall temperature after thermal equilibrium for laser tube and ambient temperature

Fig. 6. Laser preheating and locking process

Fig. 7. Beat frequency curve of laser frequency in steady state

Fig. 8. Allan variance of laser frequency in steady state

Fig. 9. Experimental result of laser frequency repeatability in steady state

Fig. 10. Laser tube temperature versus ambient temperature after frequency stabilization

Fig. 11. Shift of laser frequency with ambient temperature