Abstract
In this paper, we used a homemade external cavity diode laser (ECDL) as our seed laser and its wavelength was 778 nm. Then, we used a commercial tapered amplifier (Newport TA-7600) to boost our power up to more than 500 mW. After the tapered amplifier, the laser beam was split into two beams. One was sent to a waveguide PPLN to double the frequency of 778 nm and the other one was beaten with the optical fiber comb system in our lab. We used the beam at 389nm to perform the saturated absorption spectroscopy and measured the transition of helium atom from 2 3S to 3 3P. The absolute frequency was measured by the fiber comb system. We used the beat frequency between the ECDL and fiber comb system and an offset lock circuit to lock our laser on fiber comb system. Therefore, we could precisely know the absolute frequency of our laser and the targeted transition. The results of the transition frequencies were as follow: 23S1→33P1 f=770724725.990(235) MHz and 23S1→33P2 f=770724066.748(112) MHz. The precision of the results reached 〖10〗^(-10), but the accuracy of the absolute frequency had to be studied again and carefully because there were some effects between atoms and light, which could shift the center frequency of the transition.