Laser frequency locking based on the normal and abnormal saturated absorption spectroscopy of S7Rb

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 52S1/2, F = 2-52P3/2, F1 = 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 52S1/2, F = 2-52P3/2, F1 = 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.