First‐ and third‐derivative signals of saturated absorption in iodine 127 molecular vapor are detected by using a frequency‐modulated 190‐cm‐long 633‐nm He‐Ne laser with a Fox‐Smith–type resonator and an iodine absorption cell which is set outside of the resonator. Aiming to stabilize the laser with unmodulated output, a retroreflector‐type optical modulator is added to the above system, and the first‐derivative signal is obtained by external modulation. Comparing the obtained signal with those obtained by the frequency‐modulated laser, possibilities to detect the third‐derivative signal by using the optical modulator and to stabilize the unmodulated laser are found.