Laser frequency locking based on the normal and abnormal saturated absorption spectroscopy of ⁸⁷Rb

We present a practical method to avoid the mis-locking phenomenon in the saturated-absorption-spectrum laser-frequency-locking system and set up a simple theoretical model to explain the abnormal saturated absorption spectrum. The method uses the normal and abnormal saturated absorption spectra of the same transition 5²S_1/2, F=2-5²P_3/2, F'=3 saturated absorption of the ⁸⁷Rb D₂ resonance line. After subtracting these two signals with the help of electronics, we can obtain a spectrum with a single peak to lock the laser. In our experiment, we use the normal and inverse signals of the transitions 5²S_1/2, F=2-5²P_3/2, F'=3 saturated absorption of the ⁸⁷Rb D₂ resonance line to lock a 780-nm distributed feedback (DFB) diode laser. This method improves the long-term locking performance and is suitable for other kinds of diode lasers.