Fig. 1. Schematic diagram of detecting saturation absorption spectroscopy by third harmonics method

Fig. 2. Schematic of high output power iodine stabilized He-Ne laser

Fig. 3. Cavity length drift of iodine stabilized laser

Fig. 4. Output power drift of iodine stabilized laser

Fig. 5. Reproducibility of the output power of iodine stabilized laser within two months

Fig. 6. (a) Third harmonics error signal of iodine stabilized laser; (b) laser output power

Fig. 7. (a) Second harmonics signal of iodine stabilized laser with cavity scanning; (b) PZT voltage for cavity scanning

Fig. 8. Tracking voltage while laser locked

Fig. 9. (a) Frequency fluctuation of the f component while laser locked; (b) histogram of frequency fluctuation

Fig. 10. Frequency stability of iodine stabilized laser

Fig. 11. Frequency reproducibility of iodine stabilized laser